@Alexmesh 

Two things you can do in regard to smartphone (not) connecting to WiFi5:

1. Enable Guest Network (if it is disabled), configure it without password and with only 5GHz, see if smartphone can connect to WiFi5 open network SSID.

2. If it does not, power off all Satellite Deco, leaving just Main Deco running. In close proximity to Main Deco, see if smartphone can connect to passwordless WiFi5 Guest network.

If that does not work, you have the following: "smartphone can't connect on WiFi5 to standalone WiFi router (no mesh) even with open network (no password)." That you should take to blackview support forums.

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In regard to powerline connectivity. Yes, you are not the only one who reported the issue with PX50. This is new TP-Link product using different powerline protocol than older powerline adapters.

Which means, issues possible. If you can wait (unknown amount of time) till possible fix, just keep checking this forum for updates and beta firmware. Which might take long time to be released.

If you can't wait, follow my recommendation with standard powerline adapters. On top of that, if you are considering to try different WiFi mesh brand, same applies: find brand that supports Ethernet backhaul and connect its WiFi mesh units through standard powerline adapters. 

That solution expands options you have with powerline tremendously. You are no longer limited by just poweline capable WiFi mesh units.