Fido

There is a "+" sign to allow me to add a new APN, and I can edit any of the existing APNs by tapping on the name 

I just resolved my problem by using the settings for "Fido LTE 4G" at https://www.4gapn.com/ca/en/fido The real difference seems to be the MMS proxy setting apart from the APN protocols... NameFido LTEFido LTE 2APNltemobile.apnltemobile.apnProxyNot s...

Your workaround works for me! 

I also have a similar issue with OnePlus 6 running Android 9. My phone is using ltemobile.apn, and I'm unable to send MMS with the Textra app while connected to the Wi-Fi. I have to disconnect from the Wi-Fi before I can send MMS. There's no problem ...