Hi  @David-TP 

Th ISP Modem is a Huawei HG6145F.

I test running it ISP - M4 Main - M4 #1 and it kind of worked, on the app it showed as connected via ethernet and all good, actually I though on buying a switch to connect both M4 to the main deco when I realize that I was getting 80 Mbps out of 300 Mbps this way, so I rollback.

My new finding:

When this started I had the main deco in the kitchen, once this started failing, I delete the network, factory reset every deco using the reset botton and stablish a new network (with the same name using other device as main deco. For some reason the Deco that is in the kitchen, it is still working as main deco even when I configured other as main deco.

On the Kitchen Deco, the wifi is not working, It shows the network, you can connect but it does not give any internet, only ahppening with that one.

First Conclusion:

That Kitchen deco is broken and adding alot of stress to the network (and me) and must be replace

I'd like to hear different TS before throwing it to the trash :D