Air India — Rejected check-in though have VALID VISA

Myself and Two kids [ aged 4 and 2 ] have booked tickets with AirIndia . Booking reference:J863S
11/08/2021:Chennai -- Mumbai [AI 672] - First Leg
12/08/2021: Mumbai--London [AI 131] - Second Leg

1) First-leg, no issues, reached Mumbai on time.
2) Second-Leg, in Mumbai terminal2 Check-in counter, AirIndia staff [ Viraj ], rejected us to travel, reason was "Stayed in India More than 2 Years".
I have explained many times No such rule for "Dependent Visa". However, they didn't allow us.
3) Myself and Two kids [ aged 4 and 2 ] were struggled a lot and went through painful situation.
4)My husband, enquired with Home office, they confirmed that, she can enter into UK with this Visa.
5) Just because of their misunderstanding, we suffered a lot.
6) I need your help on this issue for re-booking and compensation.