It’s not all types of glass and it makes a difference whether or not your signal is propagating in a particular way (hence the comment about polarization).I've tried this. Near the windows is my best location and I've not moved it from original location where I had great speeds.
It is either the router or the mast. I have used LTE monitor and it isn't locked to any 5g band -_-.
I will have to try moving the router around and see if that makes a difference.