Unfortunately the bill from March 2017 will not be accepted as it is out dated. If the Welcome letter has your details, the telephone number as well as an account number then it should be accepted.
I would suggest submitting what you have and waiting on the porting authorities to advise if it will be accepted or not.
We see that your number is indeed ported successfully, are you sure you do not perhaps have another number set to divert. The number that you ported currently does not have any diverts set up.
In order for us to submit your port to be processed we require an invoice with an account number with the numbers listed which belong to you, if they are unable to send you an invoice you can request a authorization letter form them with their letter head just stating your details and numbers you wish to port.
Unfortunately we cannot proceed without this and I know it can be a headache, kindly request a auth letter from them and upload this letter as your invoice when submitting your port.