You must go step by step and make sure that something is correct, before moving on to next thing. I'd start at the router.
As a minimal config, you have outgoing NAT:
ros code
/ip firewall nat
add action=masquerade chain=srcnat out-interface=Internet
Then you need port forwarding rule. This one will work even with WAN IP from DHCP (assuming it's public and not filtered by ISP in any way). Server's LAN IP (192.168.88.254) should be either set in router's DHCP server as static lease or it can be set static on server.
ros code
/ip firewall nat
add action=dst-nat chain=dstnat dst-address-type=local dst-port=8075 \
protocol=tcp to-addresses=192.168.88.254
If there are no other rules (especially filter ones), this will forward the port. To help with debugging, add following rules:
ros code
/ip firewall filter
add action=log chain=forward connection-state=new dst-port=8075 log-prefix=\
"step2, forwarding" protocol=tcp
/ip firewall mangle
add action=log chain=prerouting connection-state=new dst-port=8075 \
log-prefix="step1, incoming" protocol=tcp
add action=log chain=postrouting connection-state=new dst-port=8075 \
log-prefix="step3, forwarded" protocol=tcp
If you connect from outside to <your public ip>:8075, you must see all three steps logged. If you do, connection was successfully forwarded and your problem is not on the router.