Cannot send or receive MMS on Sprint

I am a Sprint customer and am having trouble sending and receiving MMS. I had this problem with my previous phone (Moto X Pure), and switched to a new Pixel 4a a few weeks ago which seems to have the same problem. Both phones were purchased unlocked at retail. I have never had any problem sending or receiving SMS.

With the old Moto X, when I would receive an MMS with a photo while connected to LTE, a "Tap to download" icon would appear instead of the image in the Android Messages app, but it would never download. If I manually switched to 3G, then it would receive MMS successfully, both new messages and the messages that were originally received over LTE and couldn't download.

With the new phone I can’t connect to 3G at all, and it will not send or receive MMS while on LTE.

Looking at the system logs from an Android bugreport there are some errors when I try to send an MMS, mentioning "APN name is x.ispsn", followed by an error "Can not find valid APN". The log messages are attached.

I’m not able to check the APN settings - when I go to Settings → Network and Internet → Mobile network → Advanced, there is no Access point names option.

Since two different phones had the same problem, is it likely that this is a problem with my account rather than the phone?