Hi,
Welcome to VMware Community.
It is not a issue with using different version of ESXi host.I assume you have similar hardware and firmware.
If I unplug one of the hosts completely, the other is constant with replies.
What is the IP address you are try to ping.
1.Are you able to ping the gateway from RTO host when both the host connecting to modem?
2.Check whether any VM assigned with host IP?
3.Do you have NIC teaming in setup?