Is is possible to connect some services through Wi-Fi (without internet connection) while connected to 3G/4G? I've read a lot of questions about using Wi-Fi and mobile data at the same time to make downloads faster.

This is NOT what I am looking for.

I need to connect my smartphone to a local Wi-Fi service network. It does not have internet connection, it has only my door bell server.

At the same time my smartphone will be connected to 4G using all other internet services.

A simple "route" to my server's IP would be enough. It looks to easy to be made in a desktop but I have no idea if it is possible to be done in an Android device.

Regards,
Ian Maciel.
Not quite allowed.

In Android, the system is that, when you turn on WiFi, mobile data is switched off, or if you want and change the dual networks settings, then your phone can toggle between mobile data and WiFi when the signal strength of one us greater than the other. But that is done like this: if the WiFi strength is low, WiFi is switched off, and then mobile data is used. Once the strength is greater, it again switches WiFi on automatically, but switches mobile data off.

In short, you cannot use both simultaneously.

In the link that @lzzy mentioned, there is the mention of an app. I haven't used it, so cannot tell whether it works. However, it requires a rooted phone. You'll have to weigh the pros and cons of rooting the phone before you do so, in case you're not a developer.