AX4 AX3000 - TV is freezing

Hello.  I was hopeful when the guy from Best Buy told me this would be a five minute install - and one call to my ISP - but..well not to be.  Everything went OK...I unplugged my existing router - connected into the new router - replugged it in....and eventually all my TV's started up again - good - BUT one in particular TV would freeze all the time and my Sonos feed was spotty...and my signal in my garage - whihc is why I am doing this in the first place - did not improve. 

I called my ISP and one of the guys was very helpful and told me "yes I can help you with that...your two router signals are conflicting"..then the phone line dropped out!  So I recalled my ISP and the next tech who got my call bascially told me me to go pound sand "you messed with our equipment so you are on your own".  So I disconnected all my equipment and reconnected to the ISP router and all back to normal - save and except I got a new router that is suppossed to improve my signal.

I am not to computer literate....how should I trouble shoot this and try again?

thx