iPad only updates time automatically with phone network, but not over wifi

Your iPad has an internal clock that should accurately maintain time - this internal clock only requiring occasional correction if the clock drifts a little from actual time. As with many computer systems, your iPad automatically updates time using NTP (Network Time Protocol - over ) via whatever network it is connected.

https://en.wikipedia.org/wiki/Network_Time_Protocol

NTP uses Port 123 to communicate with NTP Time Servers. If your WiFi network or Internet Router/Firewall is blocking access to Port 123, your iPad will not be able to automatically set/correct time. In such circumstances you may need to change appropriate settings - for which you may need to refer to your WiFi Router’s User Guide, or seek assistance from your Internet Service Provider.

If necessary - and you are unable to use automatic Time settings - you can manually set the correct time on your iPad:

Settings > General > Date & Time > Set Automatically - set to OFF

You can now manually set the correct time; ensure that you also select the correct Time Zone for your country/region - as otherwise you iPad may not work properly with many online services (including Apple).