Solved.
I disabled FOTA and rebooted, then re-enabled FOTA and rebooted again, and eventually Wireguard package came alive (along with all the other ones) in the Package Manager Web UI section. So I installed it and and forcefully rebooted my RUT240, and have my shiny new Wireguard tabs in the relevant Network Status and VPN Web UI sections.
Just as a side note, after the second reboot (when the router found the new packages) my RUT240 connected in B3 instead of the usual B7, but I guess it doesn't matter.
Hope this helps anyone in similar situation.