HOW TO USE APPLE TIME CAPSULE AS SWITCH ONLY & BACKUP

Things are getting totally confusing.

In order to avoid an endless "try this and try that" approach and make things tons easier, my advice would be to reset the TC back to default settings and then use Apple's setup "wizard" to set up the TC again......so that it will automatically pick up the correct settings that it needs to connect to the ATT gateway.

If you decide to do this........

Power up the TC for a few minutes

Hold in the reset button on the back of the TC for 7-8 seconds and release

Allow a full minute for the TC to restart to a slow, blinking amber light status

Connect an Ethernet cable from one of the LAN <--> ports on the ATT gateway (there are probably 4 of them, choose any one that you wish) to the WAN "O" port on the TC

Click on the WiFi menu at the top of your Mac's screen

Look for a listing of New AirPort Base Station

Click directly on AirPort Time Capsule

When you click on Time Capsule, that will get the setup "wizard" up and running. It will take a few seconds to analyze the network, then display a screen that looks like the example below, except that you will see a Time Capsule pictured.

Network Name.......Type in a name for the Time Capsule wireless. Later, we will turn off the WiFi if wish

Base Station Name......Type in a name that you want to call the TC.......Time Capsule......for example

Password.......Type in a password that will be used for the TC wireless and base station

Verify......Type in the same password again to confirm

Click Next

The setup "wizard" will set up everything for you automatically. When you see the message of Setup Complete, click Done

Things will be working now. Post back if you do not need the WiFi function to be active on the TC and you are not sure how to turn off the WiFi on the device.

Apple does not officially support Time Machine backups to a Time Capsule over a third party network, so you won't know how well......or if.....things will work on your network until you try them out on your network.

The settings under the Internet tab look reversed to me. Normally, the router will wind up with the 192.168.200 .1 address and devices on the network will wind up with addresses like 192.168.200.2, 192.168.200.3, 192.168.200.4, etc.

In other words, the network router......your ATT gateway......is almost always automatically assigned the .1 address on most routers.

To test to see what IP address your main router....the ATT gateway......is using.....

Power off the Time Capsule temporarily

Connect a Mac directly to one of the LAN <--> ports on the ATT gateway

Log onto the Internet

Open System Preferences (the gear icon on the dock)

Open Network

Click on Ethernet on the left panel

Click Advanced at the lower right of the window

Click the TCP/IP tab at the top of the next window

Look for "Router" and note the IP address of the router.

This IP address would be the same IP address that you would assign under Router Address in the screenshot above

The fact that you will be connecting your Macs directly to the Time Capsule via Ethernet will improve your chances of things working, so things "should" work. But, "should" is not the same word as "will".

Your TC is behaving as a dumb switch.

The DNS address is irrelevant since it is only for internal TC use.. like if it needs a new firmware.. which is unlikely to happen.

The most important part you have done and putting in DNS values is for completeness only.

I haven't actually plugged the Time Capsule LAN into the gateway router LAN ethernet

When the Time Capsule is set up to operate in Bridge Mode.....Apple calls this "Off (Bridge Mode)".......all Ethernet ports on the Time Capsule act the same.....as LAN ports. So, if it is more convenient, you can connect the Ethernet cable from one of the LAN ports on the ATT gateway to the WAN "O" port on the Time Capsule.

I'm new to TC, been doing my clones with Carbon Copy Clone (for the last 20 years it seems)

CCC backups to a USB drive connected directly to a Mac will be faster and lot more reliable than Time Machine backups to a Time Capsule. Hopefully, you are not going to be replacing the CCC backups with Time Machine backups.

a little surprised how slow the Time Machine process is and how hot the TM is getting (estimated time to back up 800GB working install is over 24hrs unencrypted)

I also just got a gen 5 TC for another couple Macs - hope to get it all working seamlessly with the ATT DSL gateway on the same local network

If by Gen 5, you are saying that you have the "tall" or "tower" shaped version of the Time Capsule, you might not be aware that Apple designed this product in the 2012 timeframe and there have been no hardware improvements on the device since it was introduced in 2013. So, to be polite, we will say that the product is probably a bit "long in the tooth".

Apple officially exited the router business in April 2018 when they discontinued all the AirPort routers and Time Capsule.

are you guys saying a good TC unit (across a solid small local wired Ethernet network) is not reliable

Backing up over Ethernet to the TC would be better than trying to back up using WiFi.....but.....Time Machine is not as reliable as Carbon Copy Cloner. The TC is a "network drive", and backups to a network drive are more complicated.....with more things to go wrong.

i got my gen 5 (tall) TC on the cheap $85

The potential problem with used equipment is that you really don't know how long the product has been in service, no matter how nice the case might appear. For example, Apple introduced the "tall" or "tower" version of the Time Capsule in June 2013 and discontinued the product in April 2018. So, your Time Capsule might already have a year to 5+ years worth of use on the product.

The 3rd Gen TC was introduced in the fall of 2009 and discontinued in summer of 2011, so it may have been previously used for 8-10 years. Personally, I would not use the hard drive on this product unless I could afford to lose the data on the drive, but it's your call. On average, the useful life of this particular model was about 3-4 years, but you may have better luck.

have to add it's interesting ... and i've always had good like with my Apple routers and Airport

Having been a 10+ year contributor to the Apple Support forums, my opinion would be that the Time Capsule has been Apple's most unreliable product over the years. The AirPort Extreme and AirPort Express have been much better in terms of reliability.

Using a static IP is a good idea but you can start by using the default DHCP on the Internet tab.. it will then pick up an address from the ATT router and you can then switch over to static and use whatever address was assigned.. it will all basically just stick.

It is important to set IPv6 to link local only even if you are using IPv6 full system via the ATT router.. this was one I only recently discovered.. with Bob's help is important otherwise the TC will continually disappear from the network.

One other comment.. Time Machine (TM) is pretty unreliable.. especially to network targets.

(I see bob also gave you this warning after I started posting)

I run both TM and CCC.. TM to a TC on the network and CCC to local USB drive. TM managed yesterday to corrupt itself and delete 6 months worth of backups. This is not a big deal because CCC is very reliable.. and I tend to use TM just to experiment and it does good incremental backups and is well integrated into Mac OS.

You can use CCC to network targets but you miss out on bootable.. so clones yes.. Macs cannot boot from network drives.

Although the DHCP setting will work fine......If you want the TC to always request the same IP address from the ATT gateway, then you can use AirPort Utility to change the setting for Internet Tab / Connect Using from DHCP to Static

The DNS Servers.....8.8.8.8 and 8.8.4.4.....would make sense if you have also set up the ATT gateway to use the same DNS servers.

If you have not done this, then your network will be using the DNS Server numbers that the ATT gateway is using when a device connects to the Internet.....since the gateway is handling the routing duties on your network.