They have not reached EOL. They are still supported.
The pre-2021 plugs are notorious for dropping offline unexpectedly and also for being very finneky to install.
The reason they are a pain to set up is more a function of the new phone technology. The newer phones do not like a WiFi connection that doesn’t have internet. As soon as the phone senses there is no internet, it will immediately revert to the next available network with internet. That is why I had you turn the mobile data off. But, it may also be reverting back to your WiFi too soon.
The pre-2021 plug produces its own WiFi connection to the phone to setup. So, the phone does not like to hang onto the connection there since it doesn’t have internet.
You may want to try using Airplane Mode. That may have additional settings that keep the WiFi where it should be.
Here is a great read for the setup of these plugs by @Newshound: