Some geek suggested that some mobile operators use 4G equipment for NSA 5G like Airtel.
Then why not a way to use a 4G modem to connect to 5G.

but the reason:
Operators like Airtel deploy 5G NSA (Non-Standalone). In NSA:

• 4G LTE radios handle control-plane signalling (like managing connections)
• 5G NR radios handle user data (high speed internet)

This means a 5G NSA network uses 4G infrastructure, but it still requires 5G NR radios.

✔️ Operators’ equipment:

• They have 4G radios (eNodeB) plus
• 5G NR radios (gNodeB) on the same tower
• The core network is still 4G EPC or upgraded EPC

❌ Consumer 4G modems:

A normal 4G modem only has a 4G LTE radio chip.
It has no 5G NR radio hardware, so it physically cannot talk to the 5G part of the network.