My Router uses an ID space around 192.168.2.100-150 or something but it shouldn't matter since I've disabled it's DHCP and enabled the Pi-Hole DHCP
But if I assign the IP 192.168.3.14 via etc/network/interfaces.d the Pi-Hole becomes inaccessible
If I SSH into the pihole via it's ipv6 address it works and says it's IP is 192.168.3.14 but it can't make any IPV4 connections to any other device on the network
it works perfectly fine if I force a static IP of 192.168.2.200 (which also Isn't a IP my router would normally assign)
It depends on the network mask for the segment. 192.168.2.1 is naturally 255.255.255.0 or /24. That means that hosts on 192.168.2.1-192.168.2.255 can talk to each other without a router needing to be in the configuration. 192.168.3.0 is not in that same segment. Options are to either change the network mask to something bigger to include 192.168.2.0 and 192.168.3.0 in the same segment, or change your hosts to use 192.168.3.0/24 for their network.