Simplest way to send data from ESP STA to ESP AP

I have an Arduino ESP device using MQTT(Pubsubclient). It works perfectly.

I also have another ESP device that, for various reasons, can't reach my main WiFi router. It can, however, easily reach the first ESP. I can set the first ESP to run in AP and STA mode, and that works fine.

However, although I reckon I've read every page on the internet in the last few days, I don't seem to be able to find any simple examples of how to send a single value from my STA to my AP.

I don't want to serve up web pages or anything, I just want a simple way to send one 16 bit value approx every hour for the STA to the AP, so that the AP can then send it up to the MQTT broker.

Surely this can't be that hard...?

Any help gratefully received.

You could send a UDP message. Here is an example with a possible fix in the associated replies:

Google for other examples.