We just moved house.  We had trouble getting the Fibre connection up and running.  ISP of course immediately blamed our modem-router because it wasn't supplied by them. The ISP eventually accepted that the modem-router configuration was just fine (which I already knew ...).  However, it turned out the ONT out cable was connected to Lan 2 port on the ONT (Chorus Model 400).  Plugging it into Lan 1 immediately resolved the issue.

This has me wondering why anyone would plug the cable into the Lan2 port in the first place.  Do different ISP's use different LAN ports or something?