May not be relevant, but: We are in the process of "freshening" our family's cellphones and one we are considering is the OnePlus 6T. It has an LTE transceiver but no CDMA transceiver, and it is certified (but not sold) by Verizon for use on their network. When we fired it up, SMS worked outbound but not inbound. Turns out that Verizon still defaults to CDMA for at least some portions of SMS messaging. We called Verizon and had them enable a function called (IIRC) "CDMA-less roaming". Once that was enabled SMS instantly started working on the 6T, because SMS was then forced to use LTE rather than the CDMA default. (BTW, apparently Verizon is the last remaining carrier to use CDMA so more phones may be discarding support for it and we may be seeing this problem crop up more frequently.)
So... you might investigate if the hardware in question has a different radio setup and talk to your carrier about the specific phone model in question. Just an idea.