Sidenote: I tried updating firmware for the client, but since im remote to the setup the execution of the upload/upgrade would always fail. 

  @Virgo 

As already mentioned i can access the CPE AP without any problem even remotely.
It has no issues.
I have tried a spectrum analysis and changed channels a couple of times, but i thought maybe there were some other hints. 
 

  @pgeschke If it keeps jumping channels (disconnecting) it's probably down to DFS https://www.tp-link.com/us/support/faq/763/ and from what I've read there's nothing you can do about it, it will always drop out, I have the 710 which only uses DFS frequencies and it's law to disconnect itself. Have you tried plugging a cable into the client and connecting it to any old router set to AP mode then connecting your phone to that, do you get the same problem? I have a  similar setup with similar values as your readings on SNB noise etc which drops all the time due to jumping channels, as I assume it thinks it sees radar, although I can't find any logs detailed enough to tell me why it jumped channel like other manufactures provide.

The 'failed to get data' just means the connection is lost it will happen if reboot/turn off and on, the AP, or if the connection has dropped from the AP to the Client if see it on the client side, which will happen if drops the channel it was using due to DFS for several minutes before it allows itself to try to connect again (legal requirement for DFS)