1

What should I use? Also what file? Files do not have explanations, and uboot also does not, so I must ask these simple questions.

2

https://docs.gl-inet.com/en/3/release_notes/gl-ar750s-ext/

Pls check the release notes and firmware explaination

3

I did this. It does not say anything on this subject, so I asked here.

(You link to release notes for 750s, not 750.)