all you need to do is
1. press yourself against whatever you want to try and phase through
2. look directly downwards while still being pressed against the door
3. while still holding the W key towards the door open the menu and go to photo mode
4. press your free cam against your player
5. keep pushing your free cam directly into your player and press tab without letting go
6. repeat if it doesn't work the first time

P.S some doors will send you backwards and power armor might work a little better for this

Thanks for readin broda