route trafic back to WAN (for certain port)

Hello all!
It is possible to route (or NAT) some protocol back to WAN?
I have fiber internet and first router (from my ISP) is set to DMZ to my Mikrotik RouterBoard RB750Gr2 for certain port I need route connection back to fiber net router.
for example
Fiber net router:
public IP: 1.2.3.4
lan IP: 192.168.120.1
DMZ routed to: 192.168.120.10

Mikrotik:
WAN IP: 192.168.120.10
LAN: 192.168.33.0/24

so I want all incomming connection to 1.2.3.4:3333 which is from fiber router send to mikrotik 192.168.120.10 route back to 192.168.120.1:80

It is, because 80 port I need on my web server and from first router I havent user/password to manage it. So I need to route incomming connection to port 3333 route from mikrotik back to first router to port 80.
how can I do this?

I hope I explained it well
thank you for help.