Below is how to resolve this kind of issue based on my experience.
Login with Root account into your Synology by using one of terminal program.
Key in "net ads info -S
If you see "Failed to get server's current time!" error as below picture, keyin "synowin -updateDomain" command and enter.
After that re-join domain your Synology by using GUI mode and follow the below steps.
If you want to re-join domain first and try, just need to proceed below steps too.
Key in "net ads auth -P -S
If you do not see "Logon Server error, key in "net ads auth -P -S
Check DNS setting of your DC by keying in "net lookup dc
If you do not see any error after key in both commands, just login to Synology NAS with GUI and open Directory Services from Control Panel. You will see your Domains again.
Please key in your full domain name in
Take note all Commands are case sensitive.
I've tested on My Synology NAS Model RS2212RP+ with Firmware Version DSM 5.1-5022 Update 2.
Have a good time.
(Be knowledgeable, pass it on then)