I have also done this but when the Nokia ONT reboots after power cut, the internet on the older router connected through the LAN do not work. I have to unplug and then plug the LAN cable again in the router, then only internet works in the older router. Does anyone know how to solve this issue?
So you are using a Lan to WAN connection (2 subnets). There is no Static IP required in this case and you can keep the DHCP enabled on both routers. However you are also on double NAT which may affect online gaming. Overall it will not affect other uses.