I updated my iPad 2 to iOS 7.
Then,b-mobile data-only SIM[1] I can no longer communicate.
Upon closer inspection, it appears that it is no longer possible to save the APN.
I've tried multiple times, resetting the network, resetting everything, but it doesn't work.
So if you google a bit more,b-mobile has warned users not to upgrade to iOS 2 on iPad 4 and iPhone 7.[2] I found out that the APN cannot be saved. As a workaround, it seems that if you insert a SIM card that can make voice calls or send SMS, it will be fine.
Okay, so let's put in the SIM card for the iPhone 5... but it's a Nano SIM card. The iPad 2 is a micro SIM card, so I need an adapter, but unfortunately I don't have one with me. So I decided to try inserting the T-Mobile SIM card for the iPhone 4 that I used when I went to the US.
When I plugged it in, I was able to catch a signal from Softbank. When I checked the APN, I found that the value I had entered earlier was not displayed.
thereT-Mobile SIM [3] I removed the SIM card and put the b-mobile SIM card back in. I thought the APN would be blank again, but it was fine.
So, it looks like once you insert an SIM card capable of SMS and get a signal, you'll be fine.
Of course, this is just one example, but I hope it helps.
[1] Japan Communications b Mobile 4G High Speed Flat Rate 2GB Pair Gigabyte SIM Package
[2] About iOS 4 (2) update for iPhone 7 and iPad 7.0.0 (as of September 2013, 9)
[3] I've been using T-Mobile US Pay by the Day for years now.