Dear MikeMcQ,
thanks so much for answering, and for being kind enough to take my certainly very basic request for help seriously and treat me with respect as you did.
Indeed, I used the standard procedures which came with the Synology NAS when I first acquired a certificate. And indeed, since then I changed my configuration -- for a number of reasons (exploring the possibilities of my NAS led me into a number of dead ends), I reset my NAS to the factory settings / reinstalled the Synology operating system, and in the process, I assume, acquired new certificates (with slightly different settings).
So I assume that the warning was sent in the context of the original certificates, which I will not use, and so I probably can safely ignore them.
If that assumption proves wrong, and I end up with other warnings or even without a valid certificate, I probably will first try do get support from the Synology crowd, as you propose, and only if that did not work, I would come back to this forum.
If I may, I would suggest (I am not sure whether the relevant people are reading this) that the remembering/warning emails include some paragraph that addresses cases like mine, where the end user is perhaps not even aware that, let alone how, they acquired a certificate, and have no clue what to do when reminded to renew their certificate. This might concern several Synology users, and possibly those from other such systems. The same holds for the principal documentation pages on the letsencrypt-site.
Thanks again for your kind support!