It looks like the serial to network functionality on the BR1 Mini can only operate in server mode, not client mode. There is only one option for Operation Mode:
Hello Peplink Enginnering,
We use to do this a lot with another brand, it was a really good feature used for M2M applications (Machine to Machine) where there was a low bandwidth requirement.
We currently do not have any customers needing this, though if the need arises again it would be great to offer a Peplink solution.
Happy to Help,
Marcus