Coming back to this a couple of years later, this is more sinister… 
Firstly the uboot link to github changed : uboot-for-qca95xx/uboot-gl-ar750s-20201222-md5-901bf6ffffc79ff06aec709b669fdc18.bin at master · gl-inet/uboot-for-qca95xx · GitHub
Secondly I had no luck on windows 10 or windows 11 getting the recovery http page working. I thought I was going to have to upgrade manually in uboot via serial and tftp, but I found a work around.
Windows doesn’t seem to allow even the administrator to add entries to the arp table (arp -s [ip] [mac]) it gives an Access Denied error, but it does work with netsh
→ netsh interface ip add neighbors “” “192.168.1.1” “00-03-8F-xx-xx-xx”
Replace with the mac address of your AR750S, and in my case the ethernet adapter was “Ethernet”.
This seemed to do the trick, and I was able to upgrade uboot via (http://192.168.1.1/uboot.html). After upgrading uboot I now have access to the recovery webserver via linux.
Out of box Version : U-Boot 1.1.4-ga35cd45d-dirty (Jun 22 2018 - 12:48:24)
New Version : U-Boot 1.1.4 (Fri May 17 09:44:06 2019)
I hope this helps save someone some time I don’t know how they can claim this is an arp bug in linux 
If anything manually adding an entry to the arp table in linux would probably work as well - the claimed “correct” behaviour in Windows is likely a behaviour that was recently fixed in Windows too… since linux/macos/windows all don’t work without the manual arp entry.