Random wifi problems after weeks/months

[Firefly]

Hello,

I'm running Gargoyle on a WDR3600 and it's really great, but I have a really strange problem with my wifi:

I have several wifi clients (2.4 and 5 GHz) but sometimes (weeks, months) one random client can't connect to my wifi.
If this client was connected via 2.4 GHz the problem is solved by connecting it via 5 GHz and vice versa OR by restarting the router.
But the strange part is that the wifi is up and fully working for ALL of the other clients. Only this one client has this problem.

I'm only one room away from the router and I encountered this problem only one time, but it seems to be more frequent on my AP and another client downstairs (where the wifi signal is weaker). My smartphone had this issue several times aswell.

I tried different versions of Gargoyle but as far as I remember all of them had this problem. I'm currently running this build.

By the way: On smartphones and tablets my wifi signal is very, very weak. This wasn't always the case, 2 years ago it was great, even in the basement. But now I don't even have a signal on the ground/first floor (router is on the second floor). I was running OpenWRT back then and a few months ago I reverted back to OpenWRT to see if Gargoyle is causing this, but no, on OpenWRT it's the same weak signal. There are no other wifis nearby and I already switched channels for testing. All the same >.<

Can anyone help me? Thanks in advance.

Regards
Firefly

[ispyisail]

This wasn't always the case, 2 years ago it was great

What router were you using?

[Firefly]

WDR3600 from TP-Link, like I said.

[nworbnhoj]

I wonder if the aerials on the router are physically seated properly?

[Firefly]

Yes, they are, checked it several times.
But that can't be the cause as for the other clients everything is working completely normal, no problems whatsoever.

Or did you mean my signal strength problem? If so, it's not the cause either, unfortunately