How to get IPs/hostname from clients from different network(s)?

[yubiuser]

What do you use as router? You might be able to use EDNS(O)

Support for add-subnet option from dnsmasq (ECS/EDNS0 Client Subnet)

The ask: Add support within FTLDNS for parsing the IP address/subnet passed from another dnsmasq client via add-subnet=32,128 and potentially add-mac. Asking that FTLDNS support parsing of ECS (EDNS0 Client Subnet) data from an inbound query on the requesting side. The reasoning: I have an OpenWrt router running dnsmasq for my three client subnets. It forwards queries to my Pi-hole server, which is running on a separate VM. Traditionally, the recommendation by many in the Pi-hole community is …

In general, there might be not much chance with this setup, as all queries from the different subnets appear for pihole as if they would come from 0.13-forwarding-IP (which they are, from an IP view).

You might need to change your setup and assign IPs for each subnet to your Pi-hole device. Something like
https://ostechnix.com/how-to-assign-multiple-ip-addresses-to-single-network-card-in-linux/