It seems more like your phone either has lost its MMS configuration or Optus isn't pushing the MMS through to you properly. And now your clicking on the link with WiFi connected it's probably using that which can't reach the Optus MMS Server, so when your "outside" your off the WiFi network and it works over 3G/4G.
I would ask Optus technical support to "delete your profile of the MMSC", then check your phone has the right MMSC settings and send an MMS to yourself. This will then "Recreate your profile" and you should be good to go.
^ how do I know this? Former mobile tech support rep, and fixed things like this on a daily basis.
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