the router/modem is an Xfinity XB8 gateway, for which I do have the admin username and password. Several settings (including, notably, port forwarding) however are locked behind a mobile app I do not currently have. Plus in general I’m admittedly new to this sort of thing so I’m unsure how to properly check/troubleshoot the gateway regardless.
EDIT: At one point I mentioned in this reply how the upload speed for all devices connected to this gateway consistently never went above 42mbit/s regardless of distance, however I’ve now recalled that this was true even before my current issue started. As such, that bit is (seemingly) not relevant to the problem and I’ve removed it. I may end up with a different service provider if I end up physically moving somewhere else though, the upload speed is not exactly the best for my use case regardless.