Basically, it sits between your Unifi provided router (TP-Link, FibreHome etc) and your ONU BTU box (White Fibrehome/ZTE/Nokia box) that receives the incoming fibre signal from TM's port to your house with fibre optic cable. Refer to this guide here.
https://www.blacktubi.com/guide/make-any-ro...-work-on-unifi/
You don't strictly need the same exact model, any other modem that supports VLAN tagging on their port would work fine.

hi and welcome to xiaomi smarthome ecosystem

unfortunately, that sales man seem desperate clear old stock model because the ax3600 model already discontinued. but fortunately its global version as packaging you post in op is english

so please follow instructions from yt below




in Step 7, change the VLAN ID to something suitable for your fiber. unifi please enter 500, maxis enter 621