Hi Mario, Hubert,
I know I’m not Eric, but I have done tons of various tests and you should keep your server hard wired to the network. Ideally, you should have only the wr902ac connected to this 5ghz ssid. This will make sure your wr902ac processor is working the least as there will be least interruptions on that channel. On the other hand, make sure that your wired network is minimum 1gb on all LAN connections. Repeated tests of reducing the wired network elements to 100mb actually increases retries to the openwrt client! Don’t know why, but every option I tried gave the same result. You could enable the 2.4ghz radio and set it up as a separate said and then connect only the server to it. In theory it would be a better option then having both on the same ssid, but chances are there will be other users, devices using those 2.4 GHz frequencies and you will experience dropouts again. I would personally use the second 2.4 GHz ssid for the control device.
Hope this helps.
Boško