Traffic priority versus port 22 of the connected server

Hi all,
I need to configure my Router 951 in manner to access by pubblic IP to the remote server connected.
Now I have create a NAT in the firewall to specific Ip address to port 22, but due to high traffic on these connection the remote access is instable, I connect 1 time on 10 trials.
How I can make a priority for the traffic assigned to port 22? The ip assegned to this port is the same that all traffic flows is a HS server.
Can someone help me?
Thanks