I’ll take into view that the patents acquired as purchased and not from business combinations.
Impairment loss is recognised in the same year when it occurs.
In the next year, if there is a recovery of carrying amount of the asset, you can reverse impairment charges.
Here you gave recoverable amounts of FVLCD & VIU, but not the carrying amounts.
The reversal is recognised in P&L unless Asset is using revaluation model instead of cost model.
Finally, establish there is no impairment in the sixth year, calculate NBV after adjusting for amortisation, when there is no indication of impairment this year ie., carrying value is lower than its recoverable amount, reverse previous impairment charges.
IAS 36 & IndAS 36 & AS 28