I've bought an o2 Prepaid SIM for my Xiaomi Redmi Note 4 approximately 3 months ago, after inserting it into the mobile I received welcoming SMS containing settings for cellular data and MMS, and those worked quite adequately. Then I bought a Vodafone SIM to use with this phone, and inserted the o2 one into an iPhone. Welcoming SMS with settings arrived, and everything functioned properly as well, until I decided to stick the SIM back into Xiaomi. After doing so I received no welcoming message, so I assumed that this phone is already known and doesn't need it. However, activating mobile data yielded nothing. Even the icon in the notification bar hasn't shown up. I tried to request APN settings SMS from o2 Website, but all I could get out of it was "Ihre Anfrage konnte leider nicht bearbeitet werden, die Kundenberatung wird Ihnen gerne weiterhelfen." Configuring the APN manually hasn't changed anything even after rebooting phone to make sure the settings are applied. The Xiaomi is definitely not locked or faulty, since it functions with all sim-cards I have except for that o2 one, and o2 SIM is also likely not faulty, since it functions when sticked back into the iPhone. I read somewhere on the Internet that it may be caused by software lock on the band european cellular operators use for mobile data, but it certainly isn't the case since my Xiaomi is the international version, and it functioned with that sim before. Besides, a Qualcomm diagnostics utility suggested to remove that lock shows that the band is actually unlocked. All kinds of manipulations with changing the slot in which the sim-card is placed or disabling and enabling this and the second one, that worked as a solution for other dual-sim smartphones owners, bring nothing. All the o2 customer service could do was checking whether manually configured APN settings are correct. Perhaps anyone here would have some worthy suggestions.