I also ended up with a phone in boot loop, when fiddling with NV Items with QPST and QXDM tools. It seems that in this state, phone reboots when the modem picsk up the GSM signal. Removing SIM doesn't help, becouse the modem is still trying to register with any network for emergency call. So, what helped was to find a place with NO GSM signal from any provider, so the modem has no chance to carry any network (I'm living in small vilage at countryside, so I've had no problem to find such place). Or alternatively you can try to place the phone into something like farraday cage etc. When the phone has no chance to catch any GSM network it doesn't reboot, so then I was able to set the NV Items back to correct values and after writing them and power-cycling the phone, the reboot loop is gone and the phone works just as before. But obviously still without the european LTE support...