Most captive portals work by answering the DNS requests with the captive portal ip. This works only if the correct dns servers are configured and a lot of security features like dnssec, DOH, ... are disabled.
If you connect to the network and open firefox, it will display a toast to open the corresponding captive portals page. You can then login through that. Given that your VPN isn't blocking unencrypted connections etc.
Extrapolation of partial knowledge warning
I assume the network advertises a captive portals url and identifies you based on your MAC address.
I do construction work and travel every week. I've had this problem pop up in the last month when I connect to the hotel wifi. I just open Firefox and type in the default gateway IP and then it takes me to the login page. For whatever reason it stopped opening the page for me.