Looking back at your original statement, is it possible the credit expired?
Or you were subscribed to something e.g. £5 a month for unlimited texts that ate up your credit? I use(D) a website for the odd business task (PPH) and had a considerable surplus in my account awaiting my next job...I didn't log in for a while and they marked my account dormant, and charged me a monthly fee for the benefit, eating up over £60
I am not convinced it was data that used up all the credit
Did you do a check on the balance when you first re-insterted the SIM?
I thought wifi assist was standard with iOS 9?
and system services is all the things that don't fall under the apps - personal hotspot, system updates, game centre etc. Looking through it might indicate where large amounts of data have been used e.g. if it was a new device, it may sync a lot of accounts - mine would sync iCloud documents and probably photos the first time it was set up, which could use a lot of data