Go to Network -> Load Balancer -> Balancing Rules
Click on ADD and on the new window set the IP-ports-whatever is your criteria for distinguishing traffic and on target gateway select the gateway you prefer.
I added a rule:
Source IP: RouterIP
Source Port: 1194 (OpenVPN)
After that, I still cannot visit 1194 port from ppp0 outside. TCP no response.
And, I added a similar rule for port 443. I cannot visit https management site of router either.
And, after I added these rule, all computer without static netbalancer rule cannot access to internet.