Hi @MauroF,
I would recommend we look at a packet capture taken from the client when you are logging in to see what kind of traffic it is sending. I am not sure if protonmail.com is in any way related to Proton VPN, but if it is then I have seen some interesting behaviour in the past.
In these situations, the application has crafted its packets to look like DNS on port 53. The walled garden setting block all access except for DHCP and DNS (as these are needed to be able to see the login page). I should note that I would only expect this if some kind of VPN was in use, and even then many VPN solutions don't behave this way. In any case, taking a packet capture would be a good first step to see what kind of traffic the AP is allowing through the walled garden 😊
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