Comment by craftkiller

Instead of mDNS, they could update a DNS record for a subdomain (techno00.dev.thecompany.com, preferably under a different domain than your real one) to their local IP address and then do the DNS-01 challenge on LetsEncrypt to get a valid TLS cert for the subdomain. Then the only problem is some routers block DNS responses with RFC-1918 IP addresses, but everyone is using DoT/DoH by now, right? ... right?