RF Issue with WiFi?

So I was at a family member’s house over the holiday and found a weird issue with their AppleTV. It had huge lag issue requiring several seconds of buffer before video played. It could not play AppleMusic without stuttering. I could not stream to the TV either. All of this is new. A few month ago there were no issues with their set up at all.

The only new thing was a new router from FIOS.

Now I did not have time to do much troubleshooting but I did the usual basics to exclude the AppleTV. What I did find is if I moved the Apple TV I could make the problem worse, thus my thought is it’s RF related. Kicker is the signal is fine (4 dots) so maybe this is multi path? (Their ceiling has in ceiling radiant heating wires which was a thing in the 70s I guess.) There was no issues with cellular in the house.

This room does not have the ability to get wired, so it’s WiFi only. They have 1gb WiFi service in the same room as the router per Ookla Speed Test. Other places in the house on mobile devices are also showing considerable slowing. Thus, I am 90% certain the tech installed this new router, put it on 5GHz rather than 2.4, and left thinking all is well.

So my question is this: Drop down to 2.4Ghz and potentially slower top end speed but reliable service, or try a mesh with shorter hops so we max bandwidth but negate the multi path by optimizing the mesh nodes?

Anyone else have this issue and have insights I would be appreciative. Hoping to be back up there for the next holidays at some point.

Thanks!