Thank you for your answer, have you done a similar setup? tried it in a lab perhaps?

I just want to make sure on the process to do that,

1- I will configure the APs to MAPs and connect to either the default string or a custom one.

2- I will set them to use the 2.4GHz radios for backhaul link

3- I will then disable the 5GHz from the controller or at least disable those radios for the outdoor APs (those are the ones that are forbidden to be running in the 5GHz)

4- Finally, I will dc the ethernet cable of the APs and put them at their final place. power on, and wait for them to become online and register with the controller.

are those the correct configuration order? if so, I have a question regarding point 2.

- I know that MAPs have to use the same channel for backhaul and the power should be set for maximum. do I need to set those manually or when I do the checkbox to use the 2.4GHz, the configuration is done automatically ?

Thanks in advance for your help...

The 1500, as you already know, has two radios.  You can use one for backhaul and another for services.  Make sure you have a Root AP ready.

I know the 1500 have two radios, my question is, can it work with the 2.4 Radio only ?? to be used for backhaul and to serve clients as well.

Mea culpa.  I didn't understand your initial situation.

 to be used for backhaul and to serve clients 

Unfortunately, no.  It's either a backhaul or as a service and not both.