With the help of someone on the dappnode discord, I’ve gotten this working finally.
It’s a DNS issue, and adding the geth clients internal address to the /etc/hosts file solves the issue.
Add this
172.33.0.2 geth.dappnode
Then the connection should work using the following rpc provider : http://geth.dappnode:8545
If you do this from an external device, it works if you’re connected to the dappnode VPN (which solves the similar issue I raised in another topic about not being able to connect via my raspberrypi)