I have recently moved to Slingshot mobile from 2degrees. We have 2 devices an iphone 6 and a Lumia 930. On both devices I am having a lot of issues setting up MMS, I have also seen this issue on Spark before as well. 

On the Windows Phone I have tried 

Slingshot's APN
Skinny's APN
Spark's APN

to no effect except not being able to send picture messages. Interestingly I had to change my SMS Center Message number to Spark's to get my premium SMS services working as well. 

I thought having an iPhone might fix this but there is no access to APN settings at all under Cellular settings. I have enabled MMS on the iPhone and disabled imessage, reset network settings and still no chance to set APN.

I have called Slingshot a couple of times and they claim it is my device however this seems a bit strange as it is on 2 different OS's. 

Being on Slingshot is essentially Spark and they say on their site that all settings should be the same for Spark, however the ones on their site do not match up to the Spark ones.

Does anyone have any advice?