CalDAV support on iOS has always been a bit finicky. There's countless threads about it for providers like Gmail, Exchange, other mail servers, NextCloud, and more. You can try a few different things:

1. If you're using your full email address, try just using your username. Things like the "@" symbol in your username can cause the sync to stop or not validate at all. (At one point, iOS would change special characters to escaped characters, so the @ symbol would be seen as something totally different.) Hopefully, you can use just your username. If you can't use just your username, try disabling the calendar sync, then replacing the existing @ symbol with a new one -- just delete it and re-add it. Then re-enable the sync. Some have said that works. 

2. Try disabling calendar sync, then re-enabling it versus removing your account entirely. This can reset the sync. 

3. Make sure you're actually pushing the changes (or pulling them, I suppose) to the device and you're not looking for a manual sync. 

4. Try a trial of EAS and use that, if possible. EAS really is THE protocol for mobile synchronization. 

Hopefully one of these works out for you.