@tomdesmet
The answers is found online contredict eachother.
Where do they do that? There is only one answer - you need a firewall rule to allow access to the port your gui is listening on. The gui listens on all addresses of pfsense, if you have a rule on the interface your coming into pfsense from that allows access to that port on any of the addresss of pfsense lan, wan, opt, etc.. You would be able to access the web gui from that network.
What other possible answer could there be?
What rules do you have on the opt interface? Do you have any rules in floating? What port is your gui listening on? Mine is using 8443..