How do I login to AST2 with my iPhone or iPad?

See -> https://cee-trust.org/portal/ast2-login/

That would be a problem on the server side, you will need to contact the operator of the site.

You can try the suggestions in the thread that Raicya linked to but since you also mentioned the restart of the site in addition to the Gateway error. I wouldn't hope for a lot. On the iPhone have you tried with both WiFi and with WiFi turned off using cellular data. Have you contacted the people who operate the site?

See in this thread puneet289 response.

Those are not device errors, they are internet server errors so the problem is either on their end or it is a problem with your account. I don't use that service so I cannot log into it and it would make no difference if I did. The administrator of that site can see what is happening when you attempt to login and help you. Is there a reason you cannot do that? Have you tried logging into the site from your computer?

That is because you are new to this site, the limit expands as you participate here. It is to prevent spamming. You said at the start that this was happening both on your iPhone and iPad, that rules out the issue being on your end.

I have no idea what you mean about the site restarting. Have you contacted the people that administer the site?

That is an action on the server side. It has nothing to do with your iPhone.