It sounds like the new router might be causing the issue with port forwarding. Double-checking with your ISP about CGNAT and port settings is a good idea. Sometimes, those default settings can be tricky to navigate, but getting clarity from your ISP should help resolve the problem.

It's interesting how version 4.45.0 caused stuttering on both of your setups. Your point about the devs investigating if the update altered quality presets is spot on. Comparing it with 4.44.1 for troubleshooting was a smart move. Let's hope they address this issue soon!