Thank you to all. As I have been able to find the answer (in the Synology help forum where I didn't have to fill in any forms) Rather than start a new thread I thought I would share what I have found out.
The Synology DSM 'should' auto renew for you, but something is amiss as this is not happening. In the meantime use manual renew.
Using the DSM front end. Select Control Panel>Security. The 5th tab along is 'certificates'. Select this and there should be your ####.synology.me Certificate listed. Right click over it and select 'renew'. After a short time the listing should change with a new date 3 months in the future. This has worked for me. Fortunately I didn't need to get into the intricacies of ports and certificates from first principles.
Good luck.
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