We haven't done any updates in the last few months for the backend, so it's probably something client or network related. Have you tried to force a full reload of the site with Ctrl + Shift + R? Tried the site in an InPrivate Window? Tried Chrome? Edge is verified working here, and adblocker shouldn't be a problem unless it also blocks scripts. If the scripts are blocked, nothing will load.
Strange that Safari on iPhone doesn't trust the certificate, it's completely valid and issues by one of the largest providers (Let's Encrypt). This sounds like a caching issue on the phone, or maybe an update needed?