Time Capsule w/ Cox Gigablast

I'm not clear on whether you are asking about wired Ethernet connection speeds or wireless speeds, or both..

At best, on a really good day, using an Ethernet connection or really good WiFi connection, the 4th Gen Time Capsule might be able to handle a 325-350 Mbps connection when it is acting as a network router with a modem like the Arris SB8200. The best that I could ever get was about 275 Mbps.

What kind of performance are you seeing at the present time if you check speeds using a Mac computer?

If you are not sure which version of the Time Capsule that you have, look on the bottom of the device for the model number, usually embedded in the foam pad base. The model number starts with an "A" followed by 4 numbers. Post back with that model number if you want a bit more info on the version that you have.

Wired speeds are about 350-385 use my iMac.

Much better than I could ever get if you are talking about Internet connection speeds.

WiFi are about 85-100 with my iPhone and iPad.

iOS devices are all over the map as far as WiFi, so I would not spend much time with an iPhone or iPad as far as WiFi, as long as your connection is reliable. Remember that an iPhone or iPad is limited to a single tiny antenna that in no way compares to a WiFi antenna on a Mac.

How does WiFi look with a Mac? If the Mac is in the same room with line of sight to the Time Capsule, then you might expect 300-325 Mbps speeds assuming that the Mac connects to the 5 GHz band.

I checked by holding option and clicking on my time capsule icon.

Sounds like you are looking at the Tx Rate. This is an indication of the maximum speed that might occur between your Mac and the Time Capsule......but it tells you nothing about your actual Internet connection speed.

Run a few speed checks using a utility like www.speedtest.net How does that compare to the Tx Rate?

So my next question is if I get a different router how can I use my time capsule to keep backing up my data.

It won't be officially supported by Apple, but things should work if you connect the Time Capsule to one of the LAN <--> Ethernet ports on the new router and change the network setting on the Time Capsule from DHCP and NAT to Off (Bridge Mode). If you don't need the WiFi service from the Time Capsule, you can turn that off and use the WiFi on the new router.