No, it's not myasustor DDNS. It's a service from within my country. I had the API issue some while ago, but solved that - the DDNS provides API tokens. Actually, ADM didn't connect to the DDNS otherwise.
Who is your DDNS provider? This could mean the IP update actually failed or something, hence the process won't work.
However, I now checked the locations, there is no token in /usr/builtin/etc/certificate/letsencrypt/.well-known/acme-challenge/ and the folder /share/Web/.well-known does not exist.
As mentioned this token is temporarily created then removed. Shouldn't be the case why it failed.
No, there is only one webserver running, that's the NAS' one. The CertBot log says it cannot reach the domain: SERVFAIL looking up AAAA for my.do.main - the domain's nameserver may be malfunctioning.
AAAA records is IPv6 as opposed to A records that is IPv4. Do you have IPv6? Do a nslookup for your DDNS on both your router DNS and Google DNS 8.8.8.8. Are both the IPs (IPv4/6) correct?
Unfortunately, I cannot copy text from the log (I use Putty, and somehow copying to the clipboard just doesn't work), so I have to rewrite it here and keep it as short as possible...
Right click on the title bar, then Copy All to Clipboard.
As far as I know the Let's Encrypt will favor IPv4 over IPv6. The way both works is also different, in terms of setting up the port 80. Your error log states it seems to be using IPv6 which is strange. I have dual stack both IPv4 and IPv6, both different setups on my chroot Nginx. If you don't need IPv6, try disabling the IPv6 option in the, ADM > Settings > Network, see if it helps.
I can quickly determine the issue but It's quite tough for me to help you troubleshoot unless it's in front of me. The main thing is you need to ensure you can reach your NAS's port 80 via the DDNS. Turn on your WebServer Apache port 80. Then try `curl -Iv your-ddns.com/locale/en-US.js` see what's the reply. That 'I' is capital i. You should see some header stating Server: Apache and Content-Type: application/javascript. Also take note of its resolved IP, IPv6 or IPv4.