Also, is there any way of obtaining more detailed WAN connection logs/information, other than the limited info provided under the router's browser user interface, - under 'Status', 'Events Log'?
Yes, you can go with ssh/cli and inspecting strongswan behavior internals by do simple commands like
And RUTos offer "Troubleshoot" page tab under "System > Administration" menu.
My personal advice for filtering the problems of your question is make a test of your simcard on any other device, like usb dongle for pc or smartphone, and from this device try to ensure NORD VPN is estabilished correctly.
Some isp I know don't allow some kind of traffic trought own networks, maybe it is "crypto incompatible" or contenent inspected filtering activated by isp, so you can double check on other devices your isp "compatible" vpn configuration.
After you found working vpn setup, indeed you can apply to your RUTos.