I have an iPhone 8 and have no problem accessing this site or the forums using safari. Try clearing your history and cookies in safari in the settings.
I regularly use an iPhone 8. It failed to connect … along with everyone else using the Forums … for about a day and a half. Once that issue was fixed, I have had no further issue logging in on the phone.
There are still things I cannot do, including accessing PMs or individual user information, from the phone, and I have been given to understand that mobile-device support is no longer a Kalmbach forum development priority.
I cheat and log in by typing “trains forum” into iOS Safari, which gives me access to the Trains Magazine forum and immediately underneath, the Model Railroader forum, with Classic Trains forum being in a list of 'popular choices only a couple of list entries further down. Interestingly I do not recall having to edit any cookies on the phone to log in once the server-side issue had been resolved, and I certainly had no problem as late as this afternoon.