DHCP issues on External Interface with ISP

Question

I have been using UTM for many years now... 6+? When I first started, there were issues with certain Intel nics when used with DHCP and Cable Modems. 
 Lately, I have been having problems again. I have had MANY calls with my ISP, I even went ahead and bought new hardware for my UTM. Same problem. I rarely get an IP address from my ISP and when I do, the external interface will only stay "up" for a brief period - and then drop again. 
 I plugged my "Gateway desktop" directly into my Cable Modem and it ran without an issue for 3 days. Then I added an ASUS RT-N66 in as a "router". That ran fine for a week - with A LOT of traffic going through it. 
 I add the UTM and BAM! I am back down again. 
 My question is - WHERE do I begin to troubleshoot this? confd logs? TCPDUMP? My UTM seems happy to get a DCHP address from my ASUS if I test it that way... I don't want to bail on Sophos, but I don't know what else to do. Many years ago, I put an old router between my Cable Modem and my Sophos machine. I really don't want to do that again. 
 UTM 9.355-1 
 External NIC is an Intel Gigabit PCIe EXPI9301CT 
 Quad core i3 with 4GB RAM, 160 GB HDD 
 
 Thoughts? Advice, Similar issues?

bblank · Answer

I needed to check the "IPv4" gateway box... but that wasn't the issue. I think the underlying issue is the DHCP client settings for the external interface. It doesn't wait long enough for a response from a DHCP server. That is why every other machine or router I have can work on the cable modem but not the UTM. 
 I had my ISP change my modem back to Router mode instead of Bridged mode. So now, my modem is a DHCP client and also a DCHP server. My UTM is a DCHP client to my modem and acts as a DCHP server to my home network. I hate the "double NATing" It is similar to what I had to do years ago to get around an Intel NIC bug in Astaro. 
 I need a break from this for a little while and my family is tired of the internet connection bouncing like a rubber ball. The root cause of this could be slow DCHP responses from my ISP but good luck explaining that to them. I also just dropped $350 on a new PC for the UTM to run on which might not have been necessary, but the old hardware was 8 years old or more I think... 
 Anyway... I will be slowing down on this and letting the dust settle, but would be interested in hearing your thoughts of increasing the DHCP client "wait time" in the external NIC if that is an option. 
 Thanks for all the good troubleshooting advice. 
 Bob