Ah. Well it seems docmarius is right and I misunderstood.
You have a separate device, and you want... to connect it to one of the router's Ethernet ports(?!?). Physically speaking, of course you can.
Logically speaking - as docmarius said, if you're trying to make a local web server visible from the internet, you need to set up a dst-nat.
Assuming the web server's local IP is "192.168.88.254", the command to run from terminal would be:
/ip firewall nat add chain=dstnat protocol=tcp port=80 dst-address-type=local action=dst-nat to-address=192.168.88.254
Note that "port" can be a comma separated list of all ports you want to forward to the web server
In this setup, the public port 80 goes to the local port 80, although if you want, you could map one public port to a different local port.
Also, this setup completely hides webfig and hotspot. You'll have to also add an interface criteria if you want them available somewhere.