Applying (log backup) LSNs to the secondary node of an Availability Group
You have 2 options :


Easy one - since this is test instance and its secondary.


Stop the log backup on primary.
Remove the db/s from AG.
Take a differential backup or full backup if dbs are small.
restore then on secondary with norecovery.
take an additional log backup on primary.
restore the additional log backup taken on secondary with norecovery.
join the dbs to AG.

A little bit harder one -


install sp_restoregene on primary.
Remove the db/s from AG.
Run it for the db/s in question and it will generate the full and log backup sequence with stopat.
Restore the sequence on secondary.
Join back to AG.



(I am telling above is a little bit harder since if you have not done a point-in-time recovery and you are doing it for the first time, test it first to be comfortable).