This Apple document should help:
https://support.apple.com/en-us/HT201407
If your problem is widespread, you should consider that you may have a network routing or DNS issue. Without assistance of your network administrator, there’s little that you can do to resolve the former, but the latter is relatively easy to test.
Manually set your DNS settings to a trustworthy recursive DNS provider, such as Quad9, OpenDNS or Cloudflare.
On your iPad, you can change your DNS settings here:
Settings > WiFi > [Your Network] - touch the “i” icon, then select Configure DNS > Manual and add/replace the existing entries with either of these Recursive DNS providers - all which are trusted and highly reliable.
Quad9
Primary 9.9.9.9
Secondary 149.112.112.112
OpenDNS
Primary 208.67.222.222
Secondary 208.67.220.220
If the issue you are experiencing is occurring because of bad DNS, this change should be successful in resolving the problem.