I have now resolved this problem with the help of Synology support (whom I somewhat traduced in an earlier posting - they turned out to be extremely helpful and efficient). The issue was that the Synology network settings by default have IPv6 use set to “Auto”, and in my location (the UK) that causes problems because IPv6 is not in fact used. Setting IPv6 use to “Off” (for Synology users, go to Control Panel > Network > Network Interface > Edit > IPv6) resolved the problem and enabled me to reach LE and get a certificate issued. Thank you to all those on this forum who thought about this problem.
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