I am facing this issue which I overcame by first starting OpenVPN client connection and then adding Mullvad account number in Wireguard Client. Then I disconnected from OpenVPN and connected Wireguard and it worked on AR750s.
But this case is different for B1300 where I tried the same procedure but it never worked. I manually added settings from AR750s and copied in B1300 and named it “Mullvad” which you can see in the picture. By doing so I can only manually add one server.