TAG's preflop range: mostly 22 through 99, occasionally (if at all) maybe suited connector type hand like QJs, really can't find anything else I guess...

his turn bet is weird/unexpected, can't fold though for 22 into 55 pot.
his range at this point: block-bets with 88 or 99, not much else comes to mind...
he doesnt have low suited connector type hands in his preflop range that could connect with this board. he probably doesnt float oop with high suited connector type of hands and then donk turn. he doeesnt have a 6 in his range.

fold river. it's not tough at all. although you have to be only right like 1/3rd of the time to make this call, 77's are a bluffcatcher and he doesn't have enough bluffs in his range on that board.

disclaimer: might be biased to result (don't post result next time)

in other words:

TAG villain's preflop range is very narrow and definded here.

exercise by listing the hands which TAG-villain limp/calls in middle position, check/calls flop, donks turn and bets river on this board.

hard to find such hands, isn't it?

how many of those hands do you beat, and how many have you beat?

And which hands do you beat by the river?