Private DNS on mobile won’t let you book driving test.

In case anyone else has faced the Error 15 when trying to book a driving test, if you have a private DNS set-up on your phone to block ads, for example, it can cause you to not be able to book the test from your phone. I entirely forgot about the DNS and was trying for a few days to figure it out and then realised and it worked fine on PC, so make sure to double check if, like me, you set up the private DNS a while ago and forgot about it when trying to book.