We have a home running 1 Deco M4 unit. We noticed that some phones couldn't connect to the network or saw extremely poor signal when even 1 meter away from the router. I think I figured out what is going on. Those devices that are seeing poor performance do not support 5Ghz (like the Pocophone or some cheaper Motorola phones). By disabling 5GHz on the router (so it only sends 2.4GHz) all devices on the network (even newer ones) receive terrible signal. If I put the phone 1 foot away from the router, it will only have 1 bar. I have never seen wireless performance this poor. While I have my phone 1 foot away from the router, it has 1 bar. But it can detect wireless with 3 bars from another router I set up to test 3 rooms away with 3 bars. 

Why is the 2.4GHz so terrible on my Deco M4? I can't figure it out. I tried disabling/enabling beam forming. Is my unit defective? I can't get ahold of anyone from TP Link support. Their phone number just gives a message about covid and recommends we use the support chat and then ends the call. For the life of me, I can't find the support chat. Perhaps it was removed. The only means of support I can find are these community forums.