Android 8 (Cubot X18 Plus) cannot send MMS (but can receive, and browse internet

Hi - I'm really struggling to get a Cubot X18 Plus (running stick Android 8.0.0) to send MMS messages - they just sit on 'sending' and nothing happens (and no subsequent messages get sent either).
I can receive MMS messages, and browse the web (with WiFI off)
Both my phones are on Vodafone (my other phone is Windows) and I've swapped the SIMs over and the problem remains on the Android so I'm pretty sure it's not SIM / Account related.
I've reset the APN settings to default. I've re-registered the phone onto the Vodafone network (on 4G, 3G, and 'use preferred'). I've tried creating a manual APN using the settings on the Vodafone site. I've tried sending camera photos, saved images (from incoming MMS, so presumably of an acceptable size / format), and also the animated GIFs that come with Android. None of these seem to resolve the problem.
I've rebooted the phone in between each change to settings. The only thing I haven't tried is a full reset of the phone (which I'd rather not do unless absolutely necessary).
Anyone else seen anything similar, or have any ideas?
Can anyone confirm if any of the APN settings specifically relate to MMS outbound (and not inbound and/or web browsing?)

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