Question about MAS devices

Hi there

Recently got a sleep study in a lab done, looks like hypopnea - one of the options suggested was the MAS, vs the CPAP.

I'm finding it hard to get a lot of information on MAS devices online, but I did find a picture at one point - is it that it's essentially a mouth guard for your upper and lower jaw, and the back of the lower one is attached to the top of the upper one? So you essentially have your mouth wires shut?

I ask because I always get congested in my nose at night - always breath through my mouth - and it is looking more and more to me like using a MAS really needs you to breathe through your nose - if that is true, I will either need to get the nasal passages cleared up or just go get a CPAP so I can use a mouth attachment.

Sorry if this is said 100 times here, I just had a frustrating experience with a sleep physician and I'm more inclined to listen to other people's experiences than trust another doctor right now.