This is why I said “do not even have every-few-days Internet access”. Without some kind of recurring connectivity, certificate renewals are impossible. (The client could, in theory, help, but that’s not a thing right now.)

Systems like this are all over. Almost every IoT device is an example. More seriously, networking hardware is in this category. You can’t make connectivity to networking hardware depend on a working network with Internet access to obtain certificates without introducing a circular dependency and the possibility of making it very hard to recover from downtime.

As a result, security of network control planes is often abysmal.