AccJustForThat 10/10 of the cases I need to check manually the Gateway IP to connect manually via browser to the Wifi-login.
It sounds like you have the Internet connectivity checks turned off. Try enabling it and it should pull up the login page when you connect.
router99 Were you able to sign in on the captive portal? If not, that would be the reason. You could make sure the connectivity checks are enabled.
Having per connection MAC address randomization on means you will need to log in again after each disconnect/reconnect. Changing the privacy setting to "Use per-network randomized MAC", may help.