Hoping for a bit of knowledge and guidance from you kind folks.

I've recently changed broadband providers to PlusNet and it's had a crippling effect on my home Wi-Fi.  I live in a rather large old house built around 1890 with thick brick walls and the only wired internet access point is in a terrible spot in the house for dispersing Wi-Fi. It's not been great in the past with other providers, but now I'm seeing Wi-Fi speeds of less than 1mbps in some rooms and I rely on it for home working in the pandemic.  
 

Sadly stuck with this provider for the next 18 months and trying to invest in some more kit  to remedy the situation as best I can. 
 

The M4 seemed like a good option for pairing with my PlusNet Hub One to increase Wi-Fi coverage and speeds around the house, but I've came across this review, admittedly for the P9, which has stopped me in my tracks:

"A mesh system is supposed to have each satellite connecting to its nearest satellite, thus retaining good WifI throughout your home. The Deco system does not work like this. Each satellite connects to the primary Deco which will be the one connected to your modem via ethernet.
This means the furthest Deco satellite from your modem / primary Deco, will suffer from very poor WiFi performance because it is receiving its signal from a Deco a long way from it, rather than the Deco closest to it. This is not how mesh systems are supposed to work."

What I was hoping to achieve was better Wi-Fi coverage and speeds by locating them around the house and having them connect to each other, rather than the one by the router.  If only the latter is possible I don't think it will help my situation in any way.  As it's mostly Wi-Fi on mobile phones and tablets that I'm concerned with I wasn't planning on using them for wired Ethernet either, just better Wi-Fi. 

Can anyone confirm if this gentlemans review is accurate or if he's perhaps even more in the dark with technology than I am? Will the Deco help my situation at all or so k just need to accept that I can only have Wi-Fi in 2 rather useless rooms in the house?

Many thanks for any help.