We've seen such issues in the past with AT&T in different regions. Seeing as you've been able to pinpoint hops with signs of potential congestion issues, there is a strong chance it is the usual routing issues in play. There is a site that tracks error reporting for a lot of services. They provide outage maps for many services as well as links to online support options like social media and forums for them as well. Might be worth looking them up over at www.downdetector.com/companies.
If you just exported your files to an external/flash drive to test at another location it could still be something with your system. Didn't see Windows Defender or Firewall in the list. Could be something there to look at. But considering the history, I'd be more inclined to flush out networking issues between your system and Montreal. You can try a VPN. The free use policy with many may not provide enough for full patching, but you could run the traces to see if it gives an alternate rout and whether version check gets you into the download stage. That could give you something to push AT&T with.
It is possible router/modem issues could be in play as well. May not be practical to swap them to test though. A few things you could try though. Remove the port forwarding and disable UPnP on it also if it is on--the router's built in NAT will manage things fine. Port forwarding is more for getting around security that is blocking unsolicited inbou d traffic and can inject problems when you don't need it. The game establishes an outbound link first and the stateful inspection protocol lets the data back in and NAT directs it back to the originating device.
To that end, there might be some tweaking to the SIP firewall or other router security to loosen securty in play--if the router recently updated firmware that may need to be looked at. Also, may want to turn off IPv6 to make sure it uses NAT and rule out any potential conversion weirdness since SE is still on IPv4.
If you are indeed getting it to run by taking the system to another location and it behaves then it is something specific along the route you are taking on your AT&T connection. Again, testing with a VPN from your location would give you ammo to fire at AT&T if it works. If a VPN is able to bypass problematic routing and makes it work, that pretty much is a lock that they need to address routing issues. It also provides a means to demonstrate it on demand. That is how I ultimately got TWC off their butts last year.