So as you have discovered, this modem is not currently listed on our supported modems page. If you want to be sure both the Peplink device and modem are compatible without having them in hand to test ( ie before you buy - as you requested) then you have to pick a modem from that supported list and that is why support correctly referred you there.
That said, we go to extraordinary lengths to try and support as many modems as possible, and the process for that is to plug in an unsupported USB modem to one of our devices, connect the Peplink device to the internet via a second stable (so preferably wired) connection, turn on remote assistance on the Peplink device, download a diagnostic report and then log a ticket with support and attach the diag report.
The cellular team will then remotely connect to your Peplink device and interrogate the USB modem and in 98% of cases they will be able to write a new driver to support the modem. How long this process takes can vary depending on how busy the cellular team are when you log the ticket, and how technically challenging the new modem might be to work with, but the whole process can completed in as little as 2-3 days.
Hope that clears things up for you.