Okay, I don't have the Nokia ONT. I have a Huawei one, which had poor range but was a good modem. The range became extremely poor recently and it started throwing devices off the network, so I got my own router.
If you're getting good speeds over LAN, maybe you should contact Airtel and see if they can replace the modem. The ISP provided ONT + router combos are almost never good.
When access the internet with Ethernet LAN port the speed is always as promised , but when its on WiFi definitely the speed reduces to 40-60 % due to various factors like, the distance, inteference, direction etc. mostly 5GH is faster but limited by the device laptop or mobile) should be in 10 meters range otherwise speed gets reduced more than 50%. with 2.4GH, speed is 40% lesser than 5GH attena, though the distace it permits more.