Ring attachment

If I can't attach ring at the ceiling...is it okey to attach at the door, with door anchor, like TRX do?

Because attach at the door make different angle compare if atach at ceiling.

NO,

 

you won't be able to do almost any exercises.

 

If you really can't hang it from the celling, go outside and hang it on a tree, or maybe use door pullup bar

What kind of overhead stucture is it?  Do you have ibeams?