The Monster Manual has a strong restriction on legendary actions:
If a creature assumes the form of a legendary creature, such as through a spell, it doesn’t gain that form’s legendary actions, lair actions, or regional effects.
The Conjure Fey spell can summon a fey that takes the form of a beast:
You summon a fey creature of challenge rating 6 or lower, or a fey spirit that takes the form of a beast of challenge rating 6 or lower.
Since a fey spirit is taking the form of a beast, does this mean a Titanosaurus (from the AL-legal Beasts of the Jungle Rot product) summoned through Conjure Fey (with a level 7 slot) cannot use its legendary action?