Ah, it is a plain dhcp connection. Most likely they are registering the mac address of the router at their end.
My suggestion would be to clone the mac address of the primary router to the secondary router. That way once the bridge is setup the ISP end will still see the traffic from the mac address registered with them.
You may need a VLAN ID. Here for MTNL, I have a VLAN ID set up in the modem while putting it in bridge mode and my Asus router dials a PPPOe connection. But considering the fact that Jio uses CGNAT (Carrier Grade NAT) where everyone shares the same WAN IP, it is pointless to have Bridge mode setup as you will not be able to port forward correctly. There is no solution to this really apart from taking your own private static IP. This was the scenario back in September. However after commercial launch og JioFiber, has this scenario changed is yet to be seen. Too little info and we have to rely on everyone's trial and error posts.