It has nothing to do with priority between wifi and ethernet. It is purely a bandwidth issue. It really doesn't matter what type of media the traffic comes in on. Now in theory a ethernet port could use 1gbit and fully saturate the connection going to the router chip . This would not directly affect the wifi and you have a major design issue if you have 1gbit of traffic running. Internal to the router the wifi chip connects to the cpu/switch very similar to a ethernrt port. Even if you wanted to there is no way to give one priority over the other they are all treated the same......Ie they all try to take as much as they possibly can.
This is where you must do some more careful tests. There is a huge difference between machines in your house using all your internet bandwidth and some issue that only occurs on the wifi. If you plug a pc directly into the main router via ethernet does it have the same problem. Problems that only occur in the wifi are not caused by the other router, it would be some kind of interference. If you have issues on the main router when you are connected via ethernet also then you need to see what the cameras are doing. Maybe they are uploading too much data to the internet and exceeding your upload rate.
I have 300Mbps download and 50Mbps upload.
I have one EC3 camera conntected to second/problematic router taking 200 KB/S
I used to have stuttering problem on my main router when using my phone via wifi, but I have switched to DHCP and problem went away.
1. Test with other router
When I plug in different router as extender (TP link TD-W8961ND) problem disappears
Problem occurs with TP link AC1750 D7.
I would rather use the latter since speeds are better and range is better.
2. Test with problematic router, but with ethernet cable in my laptop
When I use the other problematic router (TP link AC1750 D7), but with ethernet cable in my laptop problem still exists.
3. Also, when old modem/router is off the problem is gone.