Well the CEO observed the car form 4:20 and the restriction ends at 5 pm. If you'd got a session at 4:20 it would have expired at 4:50, and owing to the statutory grace period no PCN could have been issued before 5 pm, by which time the restriction would have ended anyway. It can thus be taken that you derived no advantage in not obtaining a session earlier.
Still, the key point would appear to be that it is not possible to display a ticket as the council has not provided a facility for one to be provided. The traffic order is
The Havering (Parking Places) (Map Based) (Consolidation) Order 2018 which is surprisingly vague as to what the requirements are when it comes to payment.
Here is a draft representation:
Dear London Borough of Havering,
When I first parked I had poor reception so I sought a WI-FI connection to initiate a ticketless session via RingGo, unfortunately I had to download and set up the app and this took a few minutes, I attach a screenshot of my payment receipt.
As no payment is required for the first 30 minutes, and the pargkin restrictions end in any event at 5 pm, you can see I have derived no advantage and the council has not lost any income.
Furthermore the allegation on the penalty charge notice is for failure to display, but this is essentially a meaningless allegation for a location at which it is not possible to obtain anything that can be displayed, as only ticketless options are available.
In light of the above I trust you will agree to cancel the penalty charge notice.
Yours faithfully,
Make sure to upload your payment receipt and keep a screenshot of the confirmation page. The council will almost certainly issue a fob-off rejection, but we have to go through the motions.