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.
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.