The Froghemoth has a 20′ Reach Melee Attack, which if successful, then (Volos, 145):
the target is grappled (escape DC 16) if it is a Huge or smaller creature.
Grappled reduces a creature’s speed to zero, so they would not be able to move closer to the Froghemoth.
This would remove any potential melee attack unless the creature has a 20′ reach.
They can still use any sort of ranged attack without any problem.
20′ reach does not mean you can attack the Tentacle grappling you
Without any sort of given mechanic that allows you to attack the tentacle (like the Roper), adding that to the monster stat block would be a Home Rule. The RAW is that tentacle is not a viable target for the Froghemoth. Losing the reach advantage would potentially be a very big change and should change the CR rating of the Froghemoth. Comparing the two, a Roper is CR5, while the Froghemoth is CR10. It’s a more powerful creature, let it be more powerful.