@CUK0724 Choose TP-Link brand and those adapters from it that have single Ethernet port. Powerline adapters with Multiple Ethernet ports have less chances to be compatible. Make sure you buy...

@qwrty_ftw Check the following TP-Link Deco FAQ, section "Q3: Which switch models support Ethernet backhaul?": General questions about Ethernet Backhaul feature on your Deco

@HuanitaG That looks like an issue with Ethernet cable connecting Main Deco to ISP modem/router. It is possible Main Deco renegotiated 100Mbps speed instead of gigabit. Unplug that Ethernet cable...

@Manuel78 Thanks. I'll keep that in mind next time I recommend someone beta firmware for M4(V4). I was under impression only Satellite must be upgraded with beta firmware that needs to be configured...

@LS2509 POE stays for Power Over Ethernet. You will need this only if you garden room does not have electricity. If there is electric outlet in garden room and you have Ethernet cable extended to the...

@Manuel78 As a follow up comment: someone reported that just by upgrading M4 to beta code it now connects to their Satellite instead of Main. Perhaps, that firmware had better understanding and...