Old AR750S Uboot code will not load anything newer than 3.104 firmware. Unfortunately, GL iNet has removed the 3.104 firmware from their firmware page. Why, I don't know, but at least one user was able to get support to send them the 3.104 img firmware file that worked with their old version of Uboot code. After loading the 3.104 img firmware file, they were able load 3.105->3.216->4.x using the .tar file and the GL iNet GUI.
This post has the link to new uboot code:
After downloading the correct version of Uboot code, you update uboot from this page: http://192.168.1.1/uboot.html
Do not load the wrong Uboot file, or your router will become e-waste.
Note: Uboot is finicky, so try different ports on the router, try different Ethernet adapters. Windows is normally the best OSes to use with Uboot.