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 (https://drive.google.com/file/d/1mCDHfuh3evbDjJun8qkkI3PP1B-bYVwK/view) 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.