I am new to Unifi fiber broadband and the technician came to install the all-in-one wifi router where the optical fiber inserted at the bottom of the router.

I am able to retrieve the PLOAM password from the router and planning to use a standalone ONU.

Can anyone recommend ONU other than Huawei? Example: Ubiquiti?

Thanks

You can ask for @Anime4000 for a SFP+ GPON solution if you really want to ditch the AIO

Ya, I did read through his post. But his one is more advanced with business/enterprise grade router like Mikrotik.
I just want to use with opnsense

no need Mikrotik also can, I have ONT Ethernet without needing to set VLAN

OPNSENSE can, just need a compatible SFP+ NIC
any universal ONU can work, afaik Ubiquiti Ufiber Nano/Loco can work with unifi