Open these ports as incoming on the remote router. You can choose to enter your Yealinks' IP address as the "destination" on the "inbound rule" you create on the router, but if you plan to add additional extensions later I recommend just setting the source and destination as 'any'. Or, for security, set a range of IP addresses for just PBX extensions.
You should also triple check to ensure SIP ALG is NOT enabled on the remote router. When enabled this can cause some weird problems as well, and some routers come with it defaulted on.