@Kevin_Z  

Thanks for replying.

1) ISP is not even in the picture: This is a private Giga-LAN. Measured speed is 555Mbps / around 70MB/s transferring a large video file without av1200. 

I am aware of the speed that it is supposed to be like 1300 which is av1200  

49 out of 1200 is about 24 thus 24 times slower and actually only 24%  

-and then you state that 20Mbps / 2.5MB/s is normal compared to the powerline link rate ??

You do realize that this is only 41% out of 24% which is less than 10% of the 1200, right ?? 

2) Testing: Been there done that. My electrical system is probably shyte but that doesn't mean that the av1200 is running at its best  

3) Tested before and after upgrading firmware from v1 to v2: no difference. 

Still a waste of £100++ I should have bought the smallest device as long as it's only good for a PRINTER in the other room  

Warning: the AV1200 TL-PA8030P are incompatible with the TP Link Deco M5 (I only have those, but it may also affect others).

Findings:

I removed my AV600 powerline and installed 3 x TL-PA8030P. Right after this happened, I constantly lost the internet connection on all my devices that were connected to the Deco M5. Intermittendly I got internet, then the connection immediately got dropped, WLAN signal was picked up and dropped, constantly. I have multiple 5GHz capable phones and experienced this on all of them. The Mi Box 3 could not connect to the internet anymore. 

And the worst: the Ethernet backhaul between the Deco M5 does not work with the AV1200 adapters TL-PA8030P. What a disappointment. It seems that the MIMO technology used is incompatible with the Ethernet backhaul and it also affects the WLAN signal coming from the Deco M5. Totally not okay and I can now throw away all those TL-PA8030P adapters to the bin.

I re-installed my AV600 powerline adapters and immediately, all WLAN problems disappieared plus the Ethernet backhaul between my Deco M5 units immediately was established and was working again.

My advise: Do not combine the TL-PA8030P adapters with your Deco M5. And while I was installing my AV1200 powerline, I also bricked my Baseus USB-C dock, the DLP function of the dock is now broken.

@TP-Link I have checked the "Lower PLC-to-VDSL Interference Mode" switch on all my TL-PA8030P (they are all V1) and I also have updated all of them to that latest 914 firmware I was allowed. They are marked CH but does this matter? I mean I can also install someting with EUR, right?.

Okay Lower PLC switch is now off on all my powerline adapters and again, the Ethernet Backhaul to my Deco M5 does not work over the Powerline adapters, the Deco M5 in my bedroom is getting connections from my smart phones in my living room but the internet connection is not stable. I am not able to fix this with the TL-PA8030P adapters.

This is how my topology looks like:

Archer MR600 router is near the window and has LTE 4G

Main Deco unit plugged in to Archer MR600 router

First TL-PA8030P plugged in near MR600 and main Deco M5 and one connection from M5 goes into the TL-PA8030P

Second TL-PA8030P is in Home Office area, 1 cable used

Third TL-PA8030P is then in the Living Room, opposite of the MR600 and Main Deco M5. TL-PA8030P has all 3 Gbit ports used, one for the NAS, one for a wired printer and one cable connection routed through the house to the Switch in the bedroom

2nd Deco M5 is in bedroom, connected to bedroom switch

The Backhaul from the two Deco over Ethernet does not work with this config.

I have added a screenshot with the disabled settings from the TL-PA8030P

As soon as I switch all the TL-PA8030P with my AV600 TP-Link Powerline, all works fine, no issues with WiFi and Internet dropping from the M5, no issues with the Backhaul (works with the AV600 from living room to the bedroom cable connection).

Since I cannot fix the problems with the TL-PA8030P, I am now switching back to the AV600 I have :(.