New tariff but now no mobile network (no 3G/4G)

[Elp]

Please help!

i changed from one o2 tariff to another o2 tariff and now i have no mobile network at all. Only able to use wifi. 
the phone and sim are the same. 
i spoke to someone at o2 and they helped me reset network connections via iphone but still not working. 
i have been with o2 for years

[MI5]

@Elp 

Did you tick the box to receive a new sim?

If so, it is O2's currently policy to disable the current sim immediately whilst the new sim is posted out to you.

[DonnaBarber212]

My phone is still saying no service 

[MI5 · Accepted solution]

@DonnaBarber212 

Looks like you should contact customer service as they can help with that – message them on Social Media if that works best for you -on Facebook (https://o2uk.co/O2CFB) , Twitter (https://o2uk.co/O2CTW) or Instagram (https://o2uk.co/O2CIG) or call on 202 or 0344 809 0202 (contract) or 4445 or 0344 8090222 (PAYG).

[H118]

I also seem to be having the same issue. I have a notification 'android system sign into network o2-UK' Called o2 this afternoon they checked network in the area and according to them no issues with my account. Waiting for IT to contact me. But...I have just tried to view my data allowance on the app, it says I don't have a data allowance (not that I have used it all) and on the website it says 'data currently unavailable' not very helpful at all