Sophos UTM is stopping working by sudden

Question

Hi, good morning. 
 
 For 2 days in a row, I arrived in the office and we had no Internet. Both days I rebooted Sophos UTM and then the Internet was back. 
 Yesterday I thought it was an AT&T issue, because we lost the internet at the same time as AT&T had issues on their end. 
 
 Now, checking further, I see we have no logs since 2019-12-04. No changes were made in the settings recently. We have no logs for any module for the past 2 days. 
 
 Checking the graphs, there are nothing during these down time. Please see below:

Even the CPU and RAM usage has nothing to show:

Again, there were made no changes recently. Actually, the last change dates from 2019-11-20 (pattern added to the Antispam filter). 
 
 Have someone already seen this before? Any idea of what is going on?

Thanks! 
 
 EDIT: 
 By the way, we have Sophos UTM 9, running the firmware 9.605-1 (I scheduled the update to the 9.700-5 version for later today)

BAlfson · Answer

Ciao Fabiano, 
 I had a similar issue with a client in Virginia two or so years ago. The problem was a glitchy ISP connection that blipped out briefly and then the ISP's equipment failed to negotiate speed/duplex with the UTM. I think the ISP was Comcast, but AT&T often uses the same equipment. I think it was a fiber connection through a Cisco router. In any case, try #7.7 in Rulz (last updated 2019-04-17) . Any luck? 
 Cheers - Bob