Why, Xiaomi 2S mobile phone can't send MMS. . . How to solve it.

I suggest you check the following steps to investigate.

1 Check whether SMS sending and receiving are normal or whether the number of SMS center is correct. (Telecom version does not need to set SMS center number)

2 Turn on the option of always receiving MMS. (System Settings >> "All Settings" > >; "mobile network" > >; "Always accept MMS")

Ask the operator whether to open the mobile Internet service. MMS can only be sent successfully if it has the function of "surfing the Internet by mobile phone", but it will not generate traffic charges, and operators will charge according to the number of messages.

4 Whether the selected attachment is too large, operators generally have strict size restrictions on the pictures sent by MMS. Please consult the operator for details.

I suggest you change the sim card of different operators and try to send it.