Greetings. English is not my official language, so please excuse me if I write something that is not well understood.
I have a Chat Server within my local network (192.168.100.2) to which other clients from the local network connect. But on the same network there are PPPoE clients. What I want is to get the PPPoE clients to ONLY have access to the chat server on the LAN. Can this be achieved? What would be the rules to apply in the RouterOS Firewall?
If PPPoE clients use addresses from different IP subnet, then it’s entirely up to firewall configuration to allow/block whatever needed. If OTOH PPPoE clients use addresses from same IP subnet, then it might still be doable, but in which way largely depends on LAN layout and current config of your firewall / PPPoE server.
Sure it can be achieved. A caveat may be overlapping subnets if the chat server runs at private addresses (as the PPPoE clients may use the same private ranges inside their networks), so you may need to dedicate a public IP (or a CGNAT one) to the chat server to avoid some clients being “mysteriously” unable to connect, possibly with a dst-nat rule.
It is not possible to give a more precise advice without knowing the topology and current firewall rules.