I also have this problem of not being able to send or receive MMS. This started after installing firmware 31.1010.0411.138 last month. Strangely, it only affects one of the two SIMs in my phone. The Belong Mobile SIM cannot send or receive MMS, but the Optus SIM can. If I swap the SIMs from slot 1 to slot 2 and vice versa, the issue still happens - I am able to send and receive MMS on the Optus SIM but not the Belong SIM. HOWEVER, if I remove the Belong SIM from the phone, then the Optus SIM is also then unable to send or receive MMS. I tested the Belong SIM in another phone and it was able to send and receive MMS . So this issue is definitely somehow related to the most recent Zenfone 8 firmware update.