It makes little sense why it works on an offsite WiFi, but not mobile data.
I’d agree with unbuckled above, it’s a DNS issue. If your mobile device is capable, use nslookup or dig to see what responses you are getting in different scenarios. It’s possible that your VPN software is leaking DNS queries out to the mobile data provider’s DNS servers while you are on mobile data and only using the correct DNS settings when you are on wifi. Possibly look for split tunnel settings in the VPN software, as this can create this type of situation.
You can also confirm this from the pihole side. Connect to the VPN via mobile data and browse to some website you don’t use often, but is not your own internal stuff. Then open the query log on your pihole and see if that domain shows up. I’d put money on that query not showing in the pihole query log.
At least on Android (I’d assume iOS does it as well), you can set Do Not Disturb (DND) to turn on and off automatically, based on the time. You can also designate certain contacts to be allowed to bypass DND, so the phone will ring normally. I setup DND a long time ago, because I don’t want to be bothered by random shit while I am trying to sleep. However, my job is such that I might reasonably be called at 03:00 and need to roll my arse out of bed and start working. So, the number they call from is set to bypass DND. My elderly mother and brother are both similarly set to bypass. It all works out quite well and if some random marketer figures out my number and calls in the middle of the night, I don’t get woken up to talk about my car’s warranty.