Just as an FYI, I saw this post and upvoted it, despite not having run into this issue, so others might have done the same. I like to encourage visibility of posts like this so people get answers and I think it’s good for Lemmy.

Best of luck!

Wow, so ISPs can usually flash custom firmware on a 3rd party router? I’m surprised that capability exists, although I can kinda see the rationale for why it does.

Out of curiosity, if your router is able to connect with their network, why do you then need to add their router back in front of yours?