I hadn't had any luck with ROMs on Sprint either until today. I decided to restore my TWRP backup of LineageOS 14.1 official, then I re-flashed the bootloader and radio from the most recent stock build (I had been on 4.4.3 to test something). The device was not active on my account at the time, but when it booted up, I got an alert from Signal Check Pro that it had an LTE signal, and the LTE indicator was on. So then I signed on and swapped devices to try to reactivate it, and immediately went into carrier settings and tapped update network settings. I got an alert shortly after saying com.android.phone had stopped, but it appeared to work, and I had a signal and LTE indicator. There was a Lineage update showing (May 17), so I downloaded it (over LTE) and installed it. Everything has been working so far, both before and after the update: voice, LTE (all bands), SMS, MMS, and RCS. So I really didn't do anything different, it's just working now.
Note, I have had SMS problems on every stock build starting with the first 6.0.1 build, and all security releases after it. I haven't had any problems on any older Android version. What happens is it works fine for a while, but at least once a day. SMS stops working until I reboot or update network settings. That's why I installed Lineage, trying to determine if it's a bug in Android or the radio. So far, no problems, but it has only been a few hours.
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