Some are not fixable, for example if it is related to the pcb or when the chip is broken inside. If it is only resoldering of the chip I think you could find a company also in India. In principle you only need a blast pipe for this chip size hot nitrogen and a cooling for the other side of the pcb.

When a proper resoldering was already tried, then the next thing would be to replace the chip. If this is also not working to find the real fault and solve it will cost to much.

BTW what have you tried already? It could be that wrong repair could put more damage to the device.

Was the fault of the modem occasionally or once and then for ever?