Do you mean you host a server on that port and wish to have all incoming traffic end up on your game server through the LTE connection.
What is the speed of that LTE connection ??
How do you propose stopping getting your game server flooded with bots?

Or

Do you mean that you want all traffic from a subnet or singe IP address to go out LTE WAN connection??

Okay,

Lets say your LTE Route currently in place is either created by default or by you and is called LTE

Route List
ISP1 - PPPOE
ISP2 - LTE

Then add a third route which copies the existing route and adds a routing mark like below.
LTE routing-mark=gameserver

Then go to routing rules and add one.

Destination address: the IP that you indicated
Action: lookup only in table
Table: gameserver

Done!
Now anybody on your LAN that looks for that IP address will get sent to the LTE router.
This does not use the port as a selection but by destination IP address. Its quick and works.
To do it by IP and port would take more work.

However this is only valid for traffic STARTING from your LAN. So all traffic from your LAN and ALL RETURN traffic from the game server will be allowed.
If you want the game server to initiate a connection (which would be very weird) then that is port forwarding but I dont think you want that ........