YouTube and everything else are distinct services and should never be used as base to define if the things are fine or not.
GFN is way more complex and demanding than we think.
Play game on GFN isn't the same as watch a YouTube video, watch a stream on Twitch.
WiFi isn't recommended at all. Is the worst way to play cloud gaming in general. Only 5GHz is OK, but depends on how far you're from the router and the amount of walls you're from your router + interferences.
Cloud gaming works better when using a Ethernet cable.
It's really hard to tell you what is causing that problem which is preventing you from playing. GFN doesn't require any ports or any additional config to work.
What you can do, is to try your old router and check if it persists or not. Only to check if your current router is causing any problems.
GFN rarely goes "offline" or cause session problems. 99% is on user's end.