How to publish results on the Web from local MQTT no-Internet network?

[wokelio]

I'm making a project where I have a local no-Internet MQTT network (scheme below).

Devices (except for RPi 3B+) needs to work outside so I can't provide them Internet access. The purpose of RPi 3B+ node is to receive sensor data and transfer it to Web server in graphic, easy-to-analyse way.

The question is: Is it possibile to switch automatically networks by RPi 3B+ node and how can I make it?

Also if anyone could recommend solution for collecting and showing data in Web server I'd really appreciate that. I'm quite newbie and it's important to me to keep the project in not over complicated way.

Thank You in advance.

Scheme:

Web server ~ . ~ . ~ RPi 3B+ ~ ~ ~ RPi 0W ~ ~ ~ ESP32 -- sensor

~ . ~ . ~ Wi-Fi connection in network with Internet access (connection to access point with Internet)

~ ~ ~ Wi-Fi connection in network without Internet access (MQTT network)

RPi 3B+ and ESP32 - MQTT clients

RPi 0W - MQTT broker and access point without Internet

[topguy]

Buy a USB Wifi dongle and connect to both wifi networks at the same time. Save hours of blood/sweat/tears.

[B.Goode]

For a simple and reliable solution, is it out of the question to use the cabled ethernet connection on the RPi3b+ to gain access to your internet-enabled local area network?

[wokelio]

I wanted to avoid that ethernet cable thing but what I can see over the Internet it would be propably easier.

And that wifi dongle - sounds interesting, thank You very much!

PS I'm new on the forum, don't know how to anwer personally, but Thank You both very much