Take into account that ONT has a very complex management over the xPON network. Then it's very complex to replace a ONT with a router. It's not equal to a simple L1 medium access. So, the best solution it's put it in simple Bridge mode.
I hope this help.
Yeah, in fact, is like I use in this moment, the ONT connected to Mikrotik for UTP Internet entry, but I wish removing the ONT of my ISP. So, you says to me that router Mikrotik dont would to be like ONT?
I need to repeat that ONTs are more than a simple Layer1 (Transport) interface. So, it's not only a link path... the xPON network has a very complex management
. Then, the operator needs to control the ONT. Wihtout this control is impossible to have a reliable xPON network. In this scenario, the bridge mode is the most simple and reliable option
. In this case you can control all Layer2 (IP) stuff. You don't lose any functionality in this mode. Futhermore, not all ISP like to leave open the bridge mode in the ONT. So, if your ISP has this option enabled, you're lucky... you can use RouterOS as your router/firewall with full IP control. The other option is a half-bridge, with only control as Layer3 (TCP/UDP). This is the worst scenario that I don't recommend (dual NAT plus DMZ). But this a common scenario with several ISPs that enforces to use his ONT also for IP (without bridge mode).
So, you have the best environment!