Hello Sir,
I am a security researcher. I have decrypted many whatsapp files before and can easily do it with your cooperation but I am not sure if I fully understand your requirement. You couldn't restore your messages on Iphone so do you want a solution to recover the messages or decrypt the file? Recovering messages is different though not impossible. And decrypting is making the crypt12 file you have human readable. That is I can make the chats readable from your computer or any device which can open a webpage. As I said I have lot of experience in Whatsapp decryption and would do it for the least amount. Also note that the bid is for decrypting the file as you have mentioned and not recovering as it is a bit complex for Iphone but I can do that also.
Looking forward to working with you,
Regards,
Keerthi A
P.S.: Please beware that some people who tell that they can recover to Iphone would try to use simple approach and jailbreak your device(It will be very easy if the Iphone is jailbreaked) but you will lose your Iphone warranty if they do so.