Finally the local Airtel technician visited earlier today, I wasn't at home, so not sure what exactly they did, but from what my wife said, they just checked the config, made some calls, and also deleted the bridge profile that was added with the inspect element hack.
Airtel ZTE
ONT/
Router --> My
Mikrotik Router (adding one more NAT layer) --> All my devices
Since Airtel technician was supposed to be coming, I had left it as above the Airtel ZTE ONT connected to the WAN port of the Mikrotik and all my devices connecting to the Mikrotik over wired/wireless.
Anyway shortly after the technician left, the Mikrotik stopped working (could confirm from the health probes from the Mikrotik), Airtel ZTE continued to work over wireless. Once I was back home, noticed that there was no link on the port connected to the ZTE, it was as-if there was no cabled attached. Tried the other ports too, but only LAN1 established a link, so looks like LAN2-4 have been disabled.
Once cable was moved to LAN1 of ZTE, internet connectivity was restored in the original configuration.
I then configured the VLAN id and enabled PPPoE client on the Mikrotik, and it connected right away. There is no indication of any config changes on the ZTE web UI, cause only the VOIP and WAN profiles exist even now.
Anyway it appears to be a true bridge setup, cause there is no VLAN tagging or anything being done at the ZTE. All config need to be done on my Mikrotik. Even though I created the VLAN interface, the PPPoE connection was pointed to the default WAN interface itself when it worked. So the ZTE is mapping the WAN VLAN (ie 100 in my case) to the LAN1 port, and no VLAN config is required on the Mikrotik.
I had read earlier about some speed doubling which happens if you make use of both PPPoE links ie the one established on the ZTE and the one on my router. I will give it a try, I can anyway access internet from the ZTE PPPoE link on my Mikrotik using the same port.