Internet Sharing Over Ethernet
Your first screenshot shows (I think) that you have configured Computer #1 with a manually entered IP address of 192.168.70.1. It's been a while since I played with this, but I dug into some of the details of Internet Connection Sharing a while ago - see an answer I wrote back in 2014 here: https://apple.stackexchange.com/a/135163/22953.

When Internet Connection Sharing is turned on on Computer #2, that machine will have two IP addresses - one for WiFi and one for Ethernet. Each will be in a different subnet. For example: WiFi gets 192.168.61.x, and Ethernet gets 192.168.2.y.

Computer #1 will need an IP address in the same subnet as the Ethernet interface on Computer #2... so 192.168.2.z.

When I wrote that answer in 2014, it seemed likely that the Ethernet subnet here would be 192.168.2, not 192.168.70. I think if you set Computer #1 to configure "Using DHCP" instead of "Manually", I hope it should be assigned an address like that automatically.