Greetings,
my network has 3 MT routers, for example R1, R2, R3.

Facts:
- they are in same OSPF area
- R1 and R2 are connected through L2TP tunnel
- R2 and R3 are in same local network
- R2 connects to Internet via PPPoE connection
- on R3 is configured WiFi guest network (DHCP also)

I want to route traffic from WiFi guest network from R3 trough R2 to Internet but don't want that this network is visible to OSPF. That network should be only for WiFi guests and must be isolated and have Internet access.

Does anyone have an idea how to accomplish this?

Thanks

Technically you can do NAT for this guest network on R3.

But I don't see any problem in OSPF knowing the route to this network: you can always apply some restrictions in firewall.
On the other hand with NAT applied it will become a more complicated task - as all guest traffic will look to other routers as coming from the R3 itself.