So I finished the mission that rewards the ring after but my inventory was maxed out during the fight due to item drops and as such I didn't get the ring. Any way to recover it?

I doubt it, since there's a lot of bugs with that here. That being said, that sucks.

The Tidal Talisman is a better alternative though, with a longer CD though. The ring can't be recharger so it's just 5 charges and done. A cheap lesson to learn.