After facing a lot of problems with Wireguard on AR750s (with the latest firmware) I switched to MT300N setting up Wireguard on it. Earlier I raised this with the service provider (vpn.ac) but they mentioned no issue on their end.
In last 24 hours I have faced no issue with Wireguard on MT300N whereas AR750s is not connecting with the same server or if connections provide no Internet activity.
On both devices I have exactly the same setting, no custom DNS servers are set up on both. Earlier AR750s failed to connect with the main router (B1300) in repeater mode so it was connected over LAN. On the other hand, MT300N is connected to the main router in repeater mode and provides VPN transmitting over 2.4Ghz flawlessly for the last 24 hours.
I think I have been able to provide enough information to reach a conclusion, apparently, the latest firmware of AR750s requires some attention.