Pihole is great and all but naturally, it will only work for your home network. So whenever you leave your home, either with your android phone or laptop, you will have to resort to using regular ad blockers for your privacy needs. I normally use nextDNS as it has some basic blocking in place already. The problem is, if I configure my phone and laptop to use it, they will no longer use pi hole on my home network.
How do you force your devices to use pihole at home and nextDNS when you are not at home? I've seen some suggestions from people that has set up a VPN at home so they always connect to that. Are there other ways to do it?
That isn't necessary to use nextDNS as your phone's DNS server.
Setting up a VPN server at home would allow you to either join your home network remotely and thus take advantage of Pi-hole (routing all your traffic via your home network, which may slow down your downloads) or to just route DNS traffic via your home network's Pi-hole.
Neither approach would have your smartphone use NextDNS.
For that, there may be apps that allow to switch DNS servers to NextDNS based on the network connection your phone is using, but I have zero experience with those.
An alternative would be to keep using whatever your mobile network configures for DNS and employ a DNS filtering app like Blokada on your smartphone.
VPN? No. You can use dynamic DNS (there is a bunch of free providers as well) which will be frequently updated to always point to the IP address you have at home. Whenever it changes, the dynamic DNS record is updated. If you have a hosting package with any provider (like Strato), dynamic records are typically available for free.