The router can perform the role of ONU if you manage to find a SFP module that does translation from PON/GPON to the 1000BaseX / SGMII and if you manage to configure the SFP port accordingly. We have added a 1000BaseX / SGMII switch to mvneta driver (Ethernet MAC in Omnia’s SoC) in Omnia default kernel. We have also created an userspace daemon for switching the metalic PHY and SFP and it sets 1000BaseX / SGMII mode as well. However, this daemon might require some tweaking for a new module.
We have found one ONU SFP module so far and it works in Omnia SFP cage with the default kernel: MikroTik/FOXCONN S-GPON-ONU module (tested). We have more samples of GPON ONU modules, pending tests.