Hi RokuEmmanuel-D ,
Thank you for taking an interest in my issue.
To answer your questions:
I signed up for a YTTV 4K free trial in early February, before the Super Bowl.
The problem was observed immediately. 4K has never worked properly at any point since I signed up.
As I said, 4K works fine on the same TV coming from other sources. And YTTV works fine for everything that's not 4K.
The only "update" was enrolling in the YTTV free trial. I have had YTTV for years, I have had my TV for years, I have had my ISP and modem for years, and my router for several months. No changes, no problems.
No, I have no other network/internet source available, other than the two bands I have on my dual band router. I have tried both, obviously, to no avail.