I'm running Ubuntu Server 18.04.1 on a Dell 7240 laptop. I have both the ethernet and wifi connected to my network (/24). The IP addresses are reserved by the DHCP server (my Asus router) and dished out to the server fine. When I originally built this machine I suffered the same problem I'm about to descibe but I subsequently rebuilt and changed some things and the problem went away until last night when it appeared again.
The server gets the IP's from the dhcp server and can ping out and connect to the external internet, get updates etc so its routing through the gateway fine. The server can ping the gateway but the gateway can't ping the server. Moreover nothing on the rest of the LAN can ping the server, nor can the server ping anything on the LAN. Its like Ubuntu Server has suddenly turned on its firewall and is denying any and all traffic except traffic going out to the internet (I checked btw and it hasn't). Or that all ability to switch has suddenly collapsed. Its so baffling!
For extra context - this problem was happening originally and like I said I rebuilt the machine. When I rebuilt I also installed NetworkManager cli because I am more familiar with it from the desktop Ubuntu and there are more troubleshooting guides using it. Documentation and help for netplan is thin on the ground at the moment. So I changed the yaml in netplan to use NetworkManager and had a solid networking experience for a few weeks. I thought maybe NetworkManager had solved this weird issue until last night.
Thanks in advance for any help anyone can provide.