Try double clicking the Home button or swipe up from the bottom of the screen and swipe Safari upwards. Go to Settings/Safari and clear History and Website Data. Open Safari and test.
Safari - Clear the history and cookies on your iPhone, iPad, or iPod touch.
Safari website data on your iPhone or iPad - How to clear.
Are you using a VPN? Do you have access to another WiFi network where you can try? That has worked for several people.
Try a restart.
Restart your iPhone, iPad, or iPod touch
Try a forced restart.
Reset Device.
If necessary
Hard reset.
Do a backup and try a factory reset. Then restore from your backup.
Restore to Factory setting.
Backup - Restore your device from an iCloud or iTunes backup.