At work just before Christmas we had 10 new Nokia Lumia 640 phones, and we use these across 2 sites. Both of these sites have Draytek 2860n routers, that where running Firmware v3.7.8.3 - All was fine and Phones connected via wifi and no issues.
Over the Christmas shutdown i updated the firmware on the Routers to the latest v3.8.2. What happened then was anyone try to connect there Nokia phones to the wifi connection the phones would lock up, and the only way to get the phone back was to remove the battery. This happened on all the Nokia phones, but no issues with iphones ,android phones nor laptops.

I took the phone to another building that has a Netgear AP connected over LAN to the draytek, and this still caused the phones to freeze up, so it don't think it is Wireless hardware related. As a side note, the phones work fine off site on other wifi spots.

I have since changed the firmware back to v3.7.8.3 and the phones all connect and work fine now.

I don't know if it makes any difference but the phones are EE supplied phones, with wifi calling enabled.