@Emanuel9 

In theory, it should work without need for a switch. At least this is how it works with my ISP router and Deco M9/M5 mesh in AP mode.

Please double check in Deco app that your Deco mesh runs in Access Point mode. See screenshot below.

If it does, and you are interested in troubleshooting, reply here and I'll suggest simple test(s) to find if what you have is problem with long Ethernet cables or with specific brand of ISP router that you have.

@Alexandre. 
 

Thanks for answering.

I've checked everything, operational mode set as AP. Regarding the long Ethernet cables I don't think they're a problem, as I've actually set main deco with a long eth cable straight to the third floor. Speed and latency are just perfect.

I think the problem is my ISP's router.

Tried connecting directly the satellites too, result is eth backhaul works fine.

What are the main router settings culprit for these malfunctions?

The ISP's modem/router is a ZTE (the model is specific, it's called TIM Hub+).

@Emanuel9 

Check the following TP-Link article: General questions about Ethernet Backhaul feature on your Deco.

It says under Q&A, Q4: "The Deco Ethernet backhaul feature is based on the standard IEEE 1905.1 protocol."

Discussion there is for D-Link switches, but it is possible your router also does not support that protocol properly, or have it turned off somewhere.

If your ISP Tech Support can't help with that, ask them to swap your modem/router with different brand. Often, ISP carries more than one router brand. Chances are, different brand will work.

If not, perhaps your initial idea of placing switch between router and Decos could work. Get the most basic 5-port gigabit switch, unmanaged and without PoE, as long as it is not D-Link. 

I tested Ethernet backhaul with TRENDnet TEGS80 (8-port) and TP-Link SG1005 (5-port), both work fine - but my ISP router Hitron CODA-4582 supports Decos just fine, too.