You should create 2 bridges.
A local-lan bridge and a wan bridge, and group the desired ports on the correct bridge.
If you are using ether1 as your gateway (for your pppoe client) group these together with ether5 into your wan bride.
And group the rest into your local-lan bridge
Finally connect your pppoe client on the wan bridge.
ros code
/interface bridge
add name=local-bridge
add name=wan
/interface bridge port
/add bridge=local-bridge interface=ether2
/add bridge=local-bridge interface=ether3
/add bridge=local-bridge interface=ether4
/add bridge=wan interface=ether1
/add bridge=wan interface=ether5
/interface pppoe-client
set <pppoe-client name> interface=wan
Then give your exposed server a dedicated IP address given to you by your ISP (or use DHCP if that is possible)
If DHCP is not available, you should also configure your wan bridge with your desired IP address.