How to connect to my Mac through SFTP through the internet?

You do need to setup port forwarding (port mapping as Apple usually refers to it).

Here is an article on setting up port mapping in the Extreme.

https://discussions.apple.com/docs/DOC-3415

And concerning remote access using a Apple router.

https://discussions.apple.com/docs/DOC-3413

It is easy to look up what ports must be opened for SFTP.

https://www.rebex.net/kb/secure-ftp/

But there is some extra complexity here unless you have static public IP from your provider in the office.

Also for business usage VPN is better way to go generally. While SFTP is fine and secure most businesses find running VPN server better way to do things.. as it places your computer at home into the local network.

I would replace your current Extreme at least as the main router with a suitably fast vpn router.

If your AirPort was in Bridge Mode, then that indicates that you already have another router there providing DHCP and NAT service. You might call this device your "modem", but it is actually a modem/router or gateway device.

When you changed the AirPort to DHCP and NAT, you saw an error message of Double NAT appear. That means that you are trying to run two routers on the same network when you only want one device acting as the router on a network.

The Double NAT error will not allow port forwarding to work correctly even if you have the settings correct on the AirPort. The AIrPort needs to be set up in Bridge Mode again and then you can try to do port forwarding on your modem/router.

A VPN router would make things tons easier for you.

I wish there were an easier way to just straight up connect to your Mac remotely.

Back to My Mac (BTMM) worked very well until Apple killed it. Unfortunately Apple determine things like deciding that you should be using iCloud and paying them money for it and not using remote access.

If you need help with this hire a local networking company to help you. You are trying to use home routers and systems when your needs are business. Especially moving around 100x GB files over internet is not within design parameters of home systems.

Bob has pointed out that you actually don't need to do anything to your Airport. Your main router is somewhere else.

Sorry I assumed too much .. one of our first questions in posts like this is to check that your Apple router is indeed the main router.. if not you need to go back to the main router.

I am wondering if you actually control the internet in your office or it is shared? Do you have internet connection in the office which you pay for directly or comes with the building? In the later case what you are attempting might really be impossible since you do NOT own the connection.

Even if you do own the connection it would be exceedingly strange to only run IPv6. However the ISP could have you on CGNAT to limit the use of IPv4 addresses which are expensive.

Your experience with other FTP servers tells me your home connection is good with plenty of upload bandwidth. However your office setup may not have sufficient download bandwidth to match. Exactly what real world speeds do you get at home and in the office on normal Ookla speedtest? Teamviewer may not actually be the problem.

There are much better ways to do things but it does depend on your infrastructure particularly in the office which we have not treated seriously enough. Already mentioned using VPN routers.

Another option is using NAS like Synology at both ends and using their syncing ability over internet to pass files between them. This is also done using VPN.

Using a disk array that is not network but locally attached to the Mac is not going to be the best option when it comes to making fast and secure systems where internet is vital to the service.

Hmmm.. I thought this might be the case. When you have a shared setup it is difficult to get adequate services for business type setup. VPN service might still be possible. However you cannot run the server in the office.. rather you would run the VPN server in your home and use your Teamviewer to establish the link from the Mac and then pull the files rather than push them.

I still want you to run speed tests on the internet in the office as I am fairly sure on any shared system you will be throttled to fairly low speed to ensure all the other people in the building get equal access. If the download is insufficient you will still be in the same situation your are now no matter how much hardware you setup. You need to work that out first.

Also for this to work you need good upload speed at home but from your previous posts this is not the issue. I would appreciate you giving me the actual numbers though .. speedtest in both your home and office.

If there is adequate bandwidth do a simple test. Setup the SFTP server in your home and the client on the Mac in the office and using team viewer set the mac in the office to download a suitably sized file from your home to see if this works. (Turn off TeamViewer while once you get it started to free up the bandwidth). If so you can automate it using a pair of NAS as I suggested syncing to each other. Or it might be done just using VPN between the Macs. Or SFTP etc.

The speed test on my home network is 462 Mbps download, 20 Mbps upload.

You are going to have issues uploading from your home.. that is the rate determining step.

At 20Mbps it is approx. 7GB per hour.. so for 100GB it would take 14hours give or take. That is certainly going to make this slow and it won't matter what end is server or client the job will still take 14hours ..

Your lucky your workplace has a full gigabit link.

Maybe the IPv6 address can be used.. This is something I am not familiar with and I will see if someone else has a clue.

Earlier, you mentioned that you had the company's AirPort Extreme configured as a bridge. This would make sense since there is another router upstream of it that is providing you with Internet service. However, with the Extreme in bridge mode any network client being served by that "main" router would also have access to your company's network. A very non-secure situation, but that would be a topic for another post.

If your goal is to use SFTP or a VPN, or any other means to transfer files from a remote location to your company's file server, it would have to be configured on that "main" router. Will or can the building management provide you with that ability?