Thank you all very much for these explanations. I am not a technician at OpenWRT, so I learn slowly.
I finally got the AR750 to work fine. I discovered the new Mountpoints feature in Luci.
To solve my problem, I factory reset my AR750 so I have about 150 kb free. I had an old 512MB USB stick, I formatted ext4, and I use the new Luci feature in firmware 3.201 Mountpoints to do the overlay mount point … reboot … now I have 477mb extra and then I restore my settings so I can make changes and install packages. To do this, I am not connected to the Internet to keep the 150 kb free … the problem was that if I restore my previous configuration and go to the Internet, after a moment there are only 50 kb free, so they cannot be done changes at all, even in SSH.
I know that for some people that not a very difficult thing to do.
Thank you