I understand the problem you're having but I'm not sure I follow what you've tried as remedies. It's not clear to me where your modem is located.

There are two types of devices that might help:
1) Ethernet over Coax devices do just what it says. This uses coax cable for the same result as if it were cat5
2)Homeplug devices, which similarly use your house's electrical wiring to send signals.

Using these, you could have a wireless access point at the far end to originate wifi signals. You could have a different access point on each level of the house. If you do this, you need to be sure that the signal path is something like the following

Modem connected by ethernet wire to ===>Router without wifi====>connected by ethernet wire to Homeplug or ethernet over coax===>Wire===>second homeplug or ethernet devices===>connected by ethernet wire to a wireless access point (WAP) that broadcasts a wifi signal.

WAPs are sold just as standalone devices. Alternatively, you can use an old or unused router BUT FIRST you need to turn off the DHCP function of that second router so that it just broadcasts a signal and does no "Routing". Otherwise, you'll have a mess.

Running ethernet wires in any house is relatively easy IF one has proper tools. I also do not understand the complication of a wire from modem to phone jack. That should also be trivial. Electricians have even better tools to do it.

Locate ethernet jacks in various locations throughout the house. If that location cannot get Wifi, then attach an access point to the nearest ethernet port. That access point is simply another Wifi connection.

Good luck trying to get some salesman to understand what an access point is, in part, due to so many manufacturers using different phrases for the same device.

There are always multiple solutions to any question, but the devices I mentioned use existing wire runs and avoid the need for incremental effort and cost. I disagree with you, running new ethernet wiring in many houses can be difficult to impossible, my house is an example of that.

As to finding a salesman who understands what an access point is, to each his own, but I never have a problem with salesmen when buying gear from Amazon or Newegg.

I've had major problems staying connected to Wi-Fi in my home DSL network modem. At one point I've moved my DSL modem to upstairs to make networking work better and it turned out not to be so since I have multiple connections.

Network constantly disconnect: DirecTV/Cell Phone(s)/Computer(s) and Wii Console.

One Router should do the trick by keeping the ISP modem downstairs and the Router upstairs using the Ethernet 1 cat5 connection in the most central location in the upper part of home nearest most used room / window to your backyard.

Set up Guest Access through your ISP Gateway.
Split up and divide your home network between the ISP modem, Guest Access and Router. By doing this, your ISP Modem will not be over burden causing disconnection to your Home Network. I've been fighting with my Frontier Communication ISP for several months and finally got everything working after Linksys or Netgear Router was reinstalled.

Keep it simple!

ISP Modem (SSID "user name") *Toshiba*
Guest Access (SSID "Separate user name + Password") *Toshiba Guest*
Router (SSID "your user name + number") *Toshiba2*

Reset all connections in your home:
You should see all your Networking Connections (Toshiba, Toshiba Guest and Toshiba2)
TV's on Modem according to location in home.
PC on Router connection or modem location.
Cell Phones on Guests connection.

By doing this, your connectivity should improve!
Good luck, Larry.