pi-hole-android-private-dns VS gravity-sync

I'm not very familiar with Route/NAT and firewalls, I think this problem will be a problem with the routing routes on my local network? or this function is not possible? I have also used some of iptables commands in my Linux system to try to repair it without success. Because I also have a Pi-Hole Android Private DNS on another device (By Using this Guide) have the same issue, using mobile network, in the LAN network with the same server it will not connect.

I used this guide but even after forwarding the relevant ports I stil couldn't get it working.

OR I could implement https://github.com/vmstan/gravity-sync as it's last commit was a year ago but users still seem to be using it.

I run the split DHCP approach for years without issues. Having only one PiHole with DHCP Server doesn’t give you a fully redundant system. I don’t use Gravity Sync myself, but their wiki covers this case with examples: https://github.com/vmstan/gravity-sync/wiki/DHCP-Replication

the github page has a really good walkthrough and explanation

I don't have anything add to your question. But I recommend installing gravity-sync if you're going to run two piholes. It will sync up DNS entries (among other things) automatically to the second pihole when you add a entry one of them. https://github.com/vmstan/gravity-sync

If you go the 2 pihole route, I strongly recommend Gravity-Synch. https://github.com/vmstan/gravity-sync

Run Pihole on both, if DNS goes down its painful. You can sync them with https://github.com/vmstan/gravity-sync