I've had the system for 1 week now and about 3 days ago the problems started.

First thing first, the topology is:
1. M4 connected to modem in the living room (ground floor, this is the main deco).
2. M4 connected to the main deco via Gigabit Ethernet in the office (1st floor)
3. M4 connected to the office via wifi in the bathroom (1st floor). (this one is just for extending the signal to the rest of the house, and is important that it's connected to office because the signal from main is barely existent.

The problem:
We are working remotely and since we've started using more than 10 devices there have been frequent connection drop-outs.
As the drop-out happens the office deco is blinking red. I get to the main deco downstairs to check it and it has a yellow light which then turns to white and after that the internet starts working again.
While that is happening, the modem is working just fine, i also had no issue prior with the previous wifi router (Linksys EA6900), problems only started since getting the Deco units.
Another thing that often occurs after this happens is that the bathroom Deco connects to the main one which makes it have a horrible connection and it won't connect back to office one by itself or even when optimizing network, it only connects back to the office Deco when i reset it by removing power.

At first It seemed that the problem happens when there is much load on the main deco from multiple devices at the same time (500Mbps connection to the internet), but it has happened also when only my work laptop + 2 phones were used.

I've read that enabling "Fast Roaming" could cause this so i've disabled that but the issue persists. I don't know what else i could try at this point. Making the Mesh AP only (No router) is not an option since we need it to control the little one's internet time.

There is also something wrong with the data usage statistics, 4 devices are reporting 16777216TB which is just wrong, no way i could have consumed 16EB in a week, none the less on a printer, an IP camera and a robo-vacuum.

Thanks for any input.