the WiFi connection @2.4 Ghz, client mode, N, 20 Mhz width, 20 dBm, it disappears (on WDR3600 client only) every 15-30 minutes for about 30 seconds and then reappears, obviously the connection drops, it doesn't matter if there is traffic data or not.
Continuously pinging the main router to try to keep alive the connection doesn't help.
Changing wireless power (increase or decrease) doesn't help.
Changing WiFi channel doesn't help.
There are no other wifi stations around to interfere, signal is strong and stable: Signal: -51 dBm - Noise: -95 dB.
I had no problems with the same router, same network and Openwrt 18.x.x
The main router (station) has no problems at all, also, I tested another WDR-3600 client with same openwrt release connected to another main router, but it had the same behaviour,
The workaround is to enable 40 Mhz width mode (just enable 40 Mhz in General Setup, but don't enable "Force 40MHz mode" in Advanced Settings), it doesn't matter if the main router has 20 Mhz only width mode.