G900T on AT&T - works, but odd delays and data problems

I recently purchased a G900T for use on AT&T. It works and I do eventually get LTE. However, the towers are VERY slow to connect. It usually takes a minute or more for GSM service, and then it will often stay on EDGE for 5-10 minutes before it finally switches over to H, H+, or LTE.

It also seems to switch the data off completely when I make or receive a call, and then once I end the call I have to wait another 5-10 minutes for LTE to come back. Obviously very annoying.

I started out on stock OL1, and now I have CM13 with OL1 modem. Same connection problems on both. (I still have my stock AT&T S4 for comparison, and it connects immediately to the tower, and LTE works as expected. So it's not a tower issue)

I would greatly appreciate any help. Here is what I've tried so far:

• I have tried going into the *#*#4636#*#* menu, and it is set to "LTE/GSM auto (PRL)". I looked at the "Select radio band" menu, but the only options available when the radio is on are "Automatic" and "EURO Band (GSM-900 / DCS-1800 / WCDMA-IMT-2000)"
  
  It's on Automatic now. When the radio is off, I see a ton of other options, but I can't select them. And they go away as soon as the radio is back on.
  
  
• I thought the problem might be the Automatic band setting. I attempted to add additional settings to the "Select radio band" menu, based on this post (http://ift.tt/1w5NmQz)
  
  I couldn't get past the step of putting USB into "RNDIS + DM + Modem" mode. I couldn't get to that screen through the dialer, and when I tried to change the setting through the shell, this is what I found:
  
  
  Code:

  ---

  getprop | grep usb
[persist.sys.usb.config]: [mtp,adb]
[sys.usb.config]: [mtp,adb]
[sys.usb.configfs]: [0]
[sys.usb.state]: [mtp,adb]

  ---

  I tried changing the various [mtp,adb] settings using setprop. I've tried changing them to [acm,diag,mtp,adb] and [rndis,acm,diag,mtp,adb], but neither seemed to work. At least I didn't get the "diagnostic" option under add port in QPST. And I'm not even sure I'm on the right track with this.

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