So you have wired your home with ethernet cables as described here, but of course you want to add a WiFi infrastructure for your mobile devices, smart devices and IoT stuff.
Welcome to WiFi hell. Yes, there is a reason why there is a big market for extravagant WiFi solutions, advanced (and very expensive) WiFi Access Points / Routers / Repeaters, mesh stuff and buzzwords.
Wikipedia has a very exaustive page on the WiFi topic and standards. Over the decades WiFi has evolved from the original IEEE 802.11b 11Mbps to the current standards that offer huge speeds and amazing bells & whistles. Unfortunately, while the improvements have been huge, the core issues with WiFi are still well present today and not easy to overcome.
Security is nowadays not an issue anymore. The latest encryption protocols have solved that. As well, bandwith is not an issue anymore. Since 2010+ (IEEE 802.11n) WiFi offers more than enough bandwidth at low enough latency for any pratical home use, from video streaming to online competitive gaming.
But there is one last issue, which is structural to WiFi technology, that has not been solved and cannot be solved no matter what WiFi AP producers say and how many antennas they put on your new fancy, and expensive, access point.
WiFi is a radio tecnology, which relies on the propagation of radio waves. Radio waves have the peculiar tendency to be absorbed and reflected by stuff, both phisical stuff and elecro-magnetic interference. And this is a problem which gets worse at higher frequencies. Guess what? WiFi works at high requencies… 2.4Ghz, 5Ghz and 6Ghz on the latest standards.
Unfortunately, in a home setting, that can be a big problem because those frequencies will be easily blocked by thick walls, rebars, metal wires and high density objects. If you have a perfectly spherical open-space home without walls or funiture, you can expect WiFi range to be of several hundred meters. When you start putting in walls, an elecrical wiring system, plumbing, maybe floor heating, big furniture and such stuff as you usually need in your home, it's not uncommon to find out that your fancy WiFi AP cannot reliably reach the next room.
Of course this is such a big issue that there are many available solutions. The ones i can think of are:
Don't bother, they are money grabs with limited improvements at best. Yes they have high fancy numers, lots of antennas and buzzwords. They cannot solve the problem anyway.
Mesh networking consist in a network of WiFi devices capable to pickup each other signal and generate their own signal. They will take care to extend the signal from one to the other letting yiu reach much larger distances.
The main drawback is cost (they are expensive and you might require many) and performance, since by sharing one network you are actually limited by the overall troughput of all your WiFi devices at the same time.
You should think of using mesh only when you cannot absolutely run wired cables and use real WiFi repeaters.
This is the only viable solution in my opinion, which require a device capable to generate a new WiFi signal in a location where the original WiFi signal doesnt extend to. Now, since the WiFi signal does not reach that place, you will need to already have an ethernet cable there. (if you do not have wired network, mesh might be your only option).
On the market you can find many so called WiFi Repeaters or WiFi Extenders which do exactly this: plug an ethernet cable on a side, generate WiFi signal on the other.
The drawback of all the commercial solutions available on the market is that each WiFi network (so, each extender/repeater) generate a different, separated network. Even if you use the same name for all of them, your devices will still drop connection and reconnect when you move aorund and your experience will be disrupted and annoying.
There is one standard which comes into help, and it's called Fast Transitioning, but unforutnately i never found a commercial product at an accessible price point for end-user consumers that supports it.
Luckly for you, OpenWRT do support Fast Transitioning, and supports even more advanced features. See this page for more details on OpenWRT.
So the best solution i have found is to place around the house a number of WiFi Access Points, each one with OpenWRT installed on it, and enabling Fast Transitioning on them. You sohuld look for a non too expensive commercial WiFi Access Point that is supported by OpenWRT, but more details here on this.
You should start with one access point and try to see how much coverage you get. As a rule of thumb:
In some very bad cases, you might evern need one AP for each room specially for 6Ghz coverage.
Of course, this require the availability of an ethernet cable reaching each WiFi AP. See the backbone page.