No, you can't get an LE cert for an IP.
Yes, you can get multiple LE certs where all the names on them resolve to the same IP.
Yes, just stop using it.
(with the small amount of problem info provided) It seems that the NAS (or the ACME process) isn't able to properly resolve the IP to the given name.
I'd start by checking that the name was spelled correctly [not the first time a TYPO has delayed things].
Then I'd check the logs on the NAS for clues.
I suggest that Synology's forum would be a better place to get answers as to why their automatic process is not working for you. Since Let's Encrypt certificates expire every 90 days, an automatic process will increase your security and simplify your life.
Please continue to check and ask your questions on this forum as well, we have people with a wide variety of usages (such as Synology's devices) and knowledge.