Sudden Network Extender Issues

Hi everyone.

Let me preface this by saying I am not an iOS user myself, but I support Apple devices for my company's executive staff. And that is what brings me here today.

One of my users has 2 iPhones. An iPhone 6 running on Verizon, and an iPhone 6+ running on AT&T. He lives in an area where AT&T is pretty much non-existent, and Verizon gets marginal 3G/very weak LTE. For this reason, he has a network extender for each carrier in his home office. His prior phones were a iPhone 4S (ATT) and Galaxy S4 (VZW). Both of these worked perfectly on the extenders.

Both new iPhones worked as well, until about a month ago. Neither one will connect to the extenders any longer, which leads to constant dropped calls if he moves from a 6x6 area that Verizon works in. Based on the timing it appears that the iOS 8.3 update could be the cause. I checked to make sure the LTE setting is on Data Only, so that it does not attempt to use VoLTE, as I read that can cause the extenders to be ignored. And unfortunately we cannot roll his phones back to 8.2, as our Exchange server requires the iOS version to be current or else mail won't sync.

Can anyone with more iOS experience/knowledge than myself offer any insight on how to proceed from here?