TLS with a NULL cipher does, in fact, sign every message. (effectively)
Does ssh or telnet have a similar feature? Transmit in plaintext but prevent others from tampering with my box?
Apparently this is what ipsec is for, but it looks like the Wiznet device they're using doesn't have support in its IP stack.
https://www.geeksforgeeks.org/computer-network-ip-security-i...
Authentication without encryption isn't very useful, unless you sign every message.