I'm no expert but curious about this type of problem. Can I ask you:
1. Is the speed of the network good if you connect your device (or other devices, like laptop) directly to the lan point in the MBR ? Yes its good, cos the lan point is hard wired direct to the main router at ONT
2. Your MBR AP has a different SSID as the one in the DB, right ? If you move the MBR AP to another room, and try connecting the device to that AP, how is the speed like ? If I move the AP to bedroom 2, the signal is ok. But not so, for bedroom 3 cos its further away from mbr. I'm reluctant to place the AP at bedroom room 2 cos its my kid bedroom. Not sure any radiation risk
3. If you place the router in the DB and access the Wifi from other rooms, would the DB block the signal (metal casing, right ?) ? My bad, its the DB cabinet, not in DB. The signal cant reach mbr which is the furthest. signal full in living, full in br 3, drops 30% in br 2, non existent in mbr
4. I don't know what device you are connecting to the MBR AP, but do you also connect those device to the other router ? What is the speed like ? Could it be the device itself giving the problem ? I doubt. I'm using mobile phone
Just sharing... In my case, I use different devices in the MBR and MBR toilet. For some, the signal is fast but others are not good, even break. So I have a XiaoMi Repeater 2 there (and I switch it on only when I need it). Actually, there is no drop in the speed via the XiaoMi Repeater 2 even though the repeater half the speed, the device does not experience drop in speed as they are already operating only in the "half speed", if you know what I mean.