Not clear sure the problem. It's better to report this issue to tp-link support directly.

You may share the screenshot of the settings you've done

wrong direction? usually when you click one unit on the utility, it will show the direction from that unit to all the other units, is that what you sees? If a specific unit has a low speed, you may...

not so sure why, but everything works fine after you pair them all up, right?

I think it is related to the ISP, here is the ISP compatibility list of the tplink cable modem: https://static.tp-link.com/2018/201801/20180108/TC-7620_ISP_Certification_List_1476341503772b.pdf

ing file? where do you download it? can you give us a link? besides, why do you need to get that?