Remove internet access without losing LAN

I have a small cluster (all nodes run Debian 10) and need to remove the internet connections of all slave nodes. The internet cable connection connects to a computer that acts as a firewall, then, that computer sources a switch. All the cluster computers are connected to the switch. The firewall computer redirects all connections to the master node which is in the switch. The connection between nodes and the cluster is done via ssh. The internet provider changed its policy and now having all those computers connected to the same port is no longer possible. Then, what I want to do is to remove the internet access of all the nodes but keep the connections in the local network. Having internet access in only one node (the master node) is enough. I have tried many different solutions I found in forums and in the internet. Most of them suggest changing or removing the default gateway. This doesn't work because then the node becomes inaccessible from the other nodes. If someone knows how to do this I will appreciate it very much. EDIT: The masking address for the external connection is 168.176. Internally, the connection between nodes starts with 192.168. Basically, each PC goes to a port on the switch. 192.168.1.100, 192.168.1.101, 192.168.1.102, etc. You can log in to the cluster via ssh to the master and then for the type of work we use the cluster for, you need to be able to ssh to the other nodes.