@Alfkbort See the following recommendation from TP-Link: https://community.tp-link.com/en/home/forum/topic/669676?replyId=1365154

@DavidWalsh I think I have an idea why you do not see Connection Preference option for this installation, but I need more information from you to be sure. For that new setup, open its Update Deco...

@AshTheDash "Can the Deco M9 Satellite deco units connect with each other even if they don't have line of sight?" Yes, they can, but it is not guaranteed. You have to try it at your place, because...

@Nalo If you need reliable Internet connection, you should consider wired link to your ISP router. Which means Ethernet cable. If you have coaxial (TV) cables at your place, with proper adapters they...

@Prax1 Both signal strengths are important. 1. Satellite Deco signal strength tells about placement of Deco unit. If Satellite Deco signal strength is Weak, Satellite Deco unit is too far from Main...

@AshTheDash Can you update your original post with the TP-Link terminology, such as: ISP modem/router (provided by your ISP) Main Deco Satellite Deco That would make it much easier to understand. For...