Gosh same here. Hubbu and I both have new iphone XR and never had extender before these phones. 1st extender they sent me after hours of googling then another hour or so on phone with verizon they said the extender was bad and sent me a new one. Installed new one yesterday and still can not hook up. Dialed #48 and said my phone is not under LTE coverage. I am sitting 18 inches away from device and internet router.
I reset network, I had Verizon do some sort of re boot , and reset my iphone back to factory settings (which was horrible as some my apps and passwords had to be re entered and my cards were de activiated from my wallet.) and made sure my software is up to date on the iphone