Can you ping the IP address of the QNAP TVS-672XT,....?
Are you using QFinder Pro,.....?
If the QNAP TVS-672XT isn't discoverable by Ping and there are no know error messages / LEDs you might want to try using another Ethernet port on the NAS if it would be automatically assigned a new IP address. This assumes the QNAP TVS-672XT had a static IP in the first place.
Edit:
I should mention that I have had some odd things happen on my QNAP TS-653D NAS with the QNAP Firewall app. After configuration of the Firewall all was fine for a while but at some point (after a Firmware upgrade) it would rebel and would interfere with normal use, ie I couldn't log into it. There was a brief window when rebooting the NAS via the power button (presumably before the Firewall would load) that I could log in. I basically had to spam the login. If I missed it there was no way in sort of like trying to get into your PC's BIOS / UEFI. Finely I had to uninstall the Firewall app to solve the issue.
Are you using QFinder Pro,.....?
If the QNAP TVS-672XT isn't discoverable by Ping and there are no know error messages / LEDs you might want to try using another Ethernet port on the NAS if it would be automatically assigned a new IP address. This assumes the QNAP TVS-672XT had a static IP in the first place.
Edit:
I should mention that I have had some odd things happen on my QNAP TS-653D NAS with the QNAP Firewall app. After configuration of the Firewall all was fine for a while but at some point (after a Firmware upgrade) it would rebel and would interfere with normal use, ie I couldn't log into it. There was a brief window when rebooting the NAS via the power button (presumably before the Firewall would load) that I could log in. I basically had to spam the login. If I missed it there was no way in sort of like trying to get into your PC's BIOS / UEFI. Finely I had to uninstall the Firewall app to solve the issue.