In my network AR750s is connected to B1300 (main router with 5Ghz only on) wirelessly in repeater mode. AR750s transmits a different SSID, has 2.4Ghz only on and runs wireguard client.
Is it possible to use AR750s to extend WiFi coverage of B1300 over 5Ghz without disabling Wireguard on AR750s?
Yes you can.
First set up repeater on AR750S and connect to B1300ās 5G wifi. Then you can turn off AR750Sās 5G wifi (it will not turn off repeater) and only broadcast 2.4G wifi.
I am not sure if you can turn off 5G wifi on AR750S first and set up 5G repeater. But it should work.
Then, actually you can just leave the 5G wifi on.
On AR750s I had WiFi (2.4Ghz) turned on while 5Ghz was off and it connected to B1300 on 5Ghz in repeater mode. But I do not see any improvement in signals of B1300, should I even expect this WiFi coverage increase? As AR750s is having Wireguard client connected and working, will it not be another network even if I turn on its 5Ghz and set same SSID on AR750s as on B1300?
When you repeater B1300 using AR750S, your B1300 signal is not boosted. It is your AR750S sending out signals. So the correct way for extending signal is to set AR750S in extender mode, and set its SSID the same as B1300ās.
But as your use AR750S as VPN client it should create a new network, not extending current network.
Thanks alot 
Appreciate that.