The bottom line, as I understand it, is that there are precisely 2 US versions of the S9: the G960U (carrier branded) and the G960U1 (SIM-unlocked, not carrier branded). There are no technical differences, it's the same phone whichever carrier you buy it from or whether you buy the unbranded version, and the only difference between a U and a U1 is that the U would have been sold locked to a carrier and with a carrier splash screen and whatever bloatware that carrier added, while a U1 would not have any of that nonsense. Since a quick web search tells me that Consumer Cellular actually sold the s9 themselves, and there is no difference between their s9 and any other US s9 apart from the bloatware, I'm pretty sure it will work.

The most likely explanation is therefore that the "support" person's system didn't have this model listed (because it's not the model they sold) and hence they say "it's incompatible" because they don't actually know anything other than what their screen shows.

The way to confirm for yourself would be to stick a Consumer Cellular SIM in it.

 

Just what are you 'waiting' for? Given you already have the phone in question and you're already a Consumer Cellular customer (so you presumably have their SIM card), is the problem that you cannot get their service to work? Reading through your postings it sounds like you simply haven't tried to just put the SIM in and tset it up. Please clarify.

 

This is a device from 2018.
It most likely will need to be replaced shortly for any number of reasons.
(Compatibility, age and wear of components, battery issues, etc.)

However, it will still be a viable device for use on Wi-Fi.