The issue has been solved via private message. Troubleshoot file has indicated that the virtual VPN interface was not created when configuring tunnel. This is a known issue and we're aiming to have it fixed in future firmware release 7.2.
As a workaround, currently it'd be enough to execute the following commands via SSH/CLI on the router and then modify firewall settings slightly. The following commands should be executed in order:
uci set network.vpn1='interface'
uci set network.vpn1.ifname='tun0'
uci set network.vpn1.proto='none'
uci set network.vpn1.metric='100'
Once done, navigate to Network>Firewall>General settings (default window), edit the "openvpn" zone and next to the "Covered networks" section select "vpn1". Additionally, make sure "Allow forward to destionation zones" has a "wan" zone specified (screenshot). Then, save & apply settings and try to restart the VPN (turn it off and on) and see if it works after these steps. I'm also attaching a screenshot how the "openvpn" zone should look like when configured this way.
Please note that this workaround may not solve these issues if more than one VPN tunnel configured at a time. As an additional way to make sure this issue does not occur often, it's recommended to include the following option in OpenVPN configuration file if possible:
This is only useful when configuring multiple OpenVPN interfaces. Please note that uci commands must be adapted accordingly to this "dev tun0" interface. If only one OpenVPN tunnel exists then it shouldn't be necessary to assign a static virtual interface name for this tunnel.