[Santas grotty] Unable to open door from inside

This sounds like it's working as intended. The buttons below the truck require the second generator to be running in order to work, hence the need for a second player to be able to complete the puzzle.

Yes there's a second generator to find and it requires 2 people to be able to access it. 
1 Person accesses the bunker while the other goes to the cabin.
I'm not a fan of spoilers so I hope this knowledge helps you solve the puzzle but if you still can't figure it out feel free to send me a DM and I can explain it to you.