AT&T Home Base internet z700a device

Hello. I am new here trying to get the most out my AT&T home base and need some advise or help please. My problem here in the northern Michigan hills is ok phone service but not great. When I have a signal strength of -105 dBm to -115 5 yellow bar's on the device, I get okay speed of 5-15 meg download. Always a pretty slow upload of 1-2 meg.. which both are accessible as long it was always like that. Phone works fine. Now here's where it gets weird. After the device sits near a window all day the bars turn green and signal strength goes up to -92 to -99 my speeds go down to 1-3 download to .5-2.5 upload. It seems backwards to me. I was going to try to add an antenna but not if it is going to make it worse. Is it possible when it goes from yellow to green it going from 3G to 4G or is LTE a different creature or is that 4G? Any ideas of what is going on? All and all I like it a lot. I canned my Cherry Capital Communications wireless for this and it was a great decision. Any advise would be helpful. Thanks You