Do Gods and Goddesses regain their power inside dungeon? In episode 12, when Hestia walked up to Bell who was battling with the envious adventurer, she was glowing while angrily saying something like "I said stop!" - referring to the battle. Meanwhile, Hermes, overlooking them from a cliff, said that Hestia released her power.

I'm aware of the rule that Gods and Goddesses are not allowed inside dungeon. But at this point, I'm confused whether the Gods and Goddesses regain their power inside dungeons, or they possess it all the time (even outside dungeon) but they just don't use it? I don't mind if the answer contain spoilers.
They never lost it. Aside from the rule of them not being allowed in the Dungeon, it is also stated at the beginning of all three forms of the series (anime, manga and LN) that the Deities are not allowed to use their power while they reside in the human world.