@Oberott I tried everything now. No help from tp link in the forum. So I have decided to demolish the Mesh based on Tp Link and go for AVM. With this system it works. Bad reputation built up.

@Kevin_Z I tried all you proposed. No switch, connected only modem - Deco - laptop. speed 4.5 to 5 download, 10 to 12 upload. Direct on modem 93 to 96 (modem laptop!) Switched M5 from another room as...

I have a new Fritzbox AVM 7530 connected to internet as modem. I use DECO as router for my home mesh. Master is connected to modem via RJ45 and from Deco Master a second cable goes to my home network...