been there done that
 

had a dead zone in room - far away from the router - here in TV - and tried ALL the boosters, Nest and meshes etc.

finally had a local electric contractor run an ethernet cable from router to powered splitter to two pcs - - through walls and soffits etc.

worth every penny !

you'll need to go to a mesh network setup. There is a potential to extend the network with another router via the jack ina jack to jack connection which will work fine with no "basement interference" however a mesh will give you additional coverage possibilities. You'll need something like the Omni where you can connect the satellites via LAN jack and they will also mesh wifi. not cheap as the other way but it gives a lot of flexibility.

I have a 3 node TP Link X-20 mesh system which covers 5800 sq ft and costs about $200 on Amazon. It works great. You need to connect.to a separate modem. I purchased mine and save $10/month on my Xfinity internet bill.

Since we invested in an Orbi mesh wifi system, we've had great connectivity on all three floors of our 3000sqft home up north and in our 2700sqft block and stucco here in Florida. A mesh system is the answer to your problems, well, at least your WiFi problem. Lol

Weak WIFI
 

Suggets you use an ORBI WIFI Extender with two Satellite's. It is expensive. I got mine at Best Buy. The main unit plugs into your router in the basement. Then one Satellite would go on the main floor and thd second Satellite would go on your top floor. Youcan see and test the system on your snart phone using the Orbi App.

Just use the lan jack and connect an additional wifi router on the third floor. Doesn't get any easier than that.

This was my first thought- only concern is if the basement router would interfere somehow if the 3rd floor devices picks up both signal albeit weaker from basement router

MOCA is a great technology: relatively inexpensive and high speed!

Just have to be careful on the specs of the MOCA device being purchased. All of the ones I've found cannot be used with Satellite TV (Dish, DirecTV) and some cannot be used if other signals are on the wire (Comcast, etc.)

However, you can isolate the cable in the box in the garage and it will work perfectly.

Powerline Ethernet Adapters are also quite good. I'm using them and getting 200 MB/s.

FWIW.... In my Lantana my wifi was so weak on the Lanai that it was about useless. I went to Best Buy and bought an extender, plugged it in by the sliders close to the Lanai and have full bars on the Lanai now. NO issues. The modem/router is in the front room. The electric outlets could care less the device is one whatever circuit. It works by taking the weak signal and boosting it using two antenna. One brings the signal in and one sends it out. Easy Peazy...

This is what I use;
Access Denied.

Weak Wi-Fi in the Home
 

You need to add a WiFi Network Mesh system. You can do it for less than $200 and it will come with a device for each floor. It plugs into your current modem and broadcasts the signal to devices on the different floors. This will get rid of all the dead spots in the house and give you the coverage you are looking for without running wires. Eero is a very popular brand and works flawlessly. There are many other good brands including one by Google. Good luck.

Although some companies sell a combo modem/router, they are generally two separate boxes.

The best and simplest solution is to buy a MESH 2-pack router system such as the Google WiFi (Best Buy link Access Denied). These are much easier to setup and maintain than a adding WiFi extender (comes with an easy phone app for setting it up; automatically installs security updates).

Keep you modem in the basement. Disconnect & discard you current router. Connect the modem to the input of the primary MESH unit via a short LAN cable. Connect the output of this primary MESH unit (in the basement) to LAN cable that runs upstairs. Then connect the 2nd MESH unit to the upstairs LAN jack. That should get you excellent coverage. If the WiFi signal is still weak in part of the house, you can place a 3rd MESH unit in that location and let it link wirelessly to the other MESH units.

This is the exact configuration I put in my 2-story house with basement, and then brought with me when I moved down here.

Good Luck

My modem has combined router

I think that would double the cost of your internet service because you can only have one modem/router.

I had limited WiFi reception within my Villages stucco home. MMD Computer in Lady Lake, recommend and installed, for a reasonable price, a device called UniFi. It connects to an app on the computer. It is the same system hospitals and large offices use where people have to bring their laptops from room to room. It works great, much better than the range extenders or boosters that plug into an outlet. Get the facts from MMD in Lady Lake. Perhaps they can sell you the equipment and you can install it yourself up north. Good luck!

Untrue
 

You can have only one modem, the unit that connects to the cable company's incoming cable. You can have as many routers as you want, each one will connect to an Ethernet port on the modem.