Look at ADVANCED | Firewall & DMZ !
It provides the opportunity to configure firewall rules. I guess you have to specify one for inbound TCP port 9891 (Source WAN 0.0.0.0 - 255.255.255.255, Dest LAN 192.168.0.101 port 9891, proto TCP --- or perhaps "Dest LAN 175.101.67.138 port 9891, proto TCP" if the rule is effective before NAT is done)