OK, so you input the IMEI on your carrier's website to swap devices from Android to an iPhone 7 Plus. Understood.
I am going to now speak some tech stuff however will also summarize below so you will know what I am talking about. The iPhone 7 Plus came in two versions CDMA & GSM and the other was GSM only. It appears that you may have the model that is GSM only and you are trying to activate on a CDMA cellular provider's network. For example in the US, it would be like activating an AT&T (GSM) iPhone 7 Plus on a Verizon network (CDMA).
I recommend that you call your carrier and clarify what I stated above to be 100% certain. Thanks.
Axel F.