Spectrum Mobile: Can’t send pictures or anything other than text to Android from iPhone

I signed up for cellular service with Spectrum Mobile in May using my iPhone 11 Pro, which I bought directly from Apple. I’d earlier been using the phone with various carriers overseas and AT&T with no problem. Spectrum was better in terms of cost but I’m unable to send pictures or anything other than text message (for example, I can’t share contacts) to Android users. There is no problem with other iPhone users.

Spectrum mobile service is awful. I’m usually on hold for 40 minutes or longer and the customer service reps who answer have no idea about technical maters and put on hold again while they try to find someone in technical support. I’ve been advised to check if I have MMS selected, which I do and they say they will ‘reset’ my phone but have no information. It seems that iPhones may not be supported by Spectrum, which is a Verizon reseller. Does anyone have experience with an iPhone with them or suggestions for how to change my iPhone configuration?