I upgraded my MiFi 5510L for a 6620L because of the better antenna. Immediately started getting IP address conflicts, perhaps 5-10 daily. The Verizon hotshots were clueless. So on the last day of the return I changed back to the 5510L and CC was credited. No more problems with the 5510L. This problem only occurred on my W7 and not on DWs 8.1 or tablet and there were 2 wireless printers. Everything up to date.
The only idea I had was I changed IPv4 for my solar charger on the W7 PC which was not connected during the above (cable to the charger not plugged in).