Usually low budget horror movies tend to be the better ones because of their creativity and creepy tone. Take your pick: Evil Dead (1981), night of the living dead (1968), Elm St (1984), and even last year's Barbarian. These movies rely of a setting, decent cast, creepy music, and some kind of scary reveal. Bunker has potential but bombs.
I thought maybe the slow pace was equal to a slow burn that pays off in the end. This is like watching a bad M. Night Shamalan movie. No pay off and just a plain dumb ending, especially when the monster is revealed. The audience I saw this with laughed at the ending and you just might too when you see what I'm talking about.
Disappointing because the setting could have worked. WW1 soldiers hide in bunker behind enemy lines during war. Their lives at stake causes them to duck into the bunker unaware there's something very spooky in their. That's all I'm gonna say in case you're still curious. I'll admit the first act did draw my attention but then after that I got so bored at one point I thought about walking out. As for the big reveal, well when you see it you may feel cheated.
You could wait for Redbox or Netflix on this one. Save the $11 ticket fee and get a pizza instead.