2 DSL Modem/Router, how to extend range?

Run a speed test with your laptop connected to the modem with an Ethernet cable.
Wired should give your best speed. Your baseline.

http://www.speakeasy.net/speedtest/

Then run the speed test with the laptop connected via wireless. Test at various ranges and locations.

Try a using a utility like Xirrus Wi-Fi Inspector to scan the airwaves for other access points.
This will also show your signal strength.

Maybe a channel change of the router will help.

You may need to add a bridge if there is a significant drop in signal within the area of use.

There are two types of bridging when you speak of wireless internet access.

1. Bridging your modem:
The router functions in the modem are turned off per the instructions from your ISP.
The modem becomes a "pass through" device.
You then connect a better quality wireless router to the modem and the router is assigned the IP address
and undertakes all the router functions for your local network.

2. Wireless bridge:
Is a device that is added to a wireless network to extend the range of the network.
It connects to the existing wireless access point and handles the connections of the
wireless computers closest to it. Bridging the connections to the router.

Example: Access point on one side of your house, wireless bridge on the other side of the house.
Full signal strength all around your house.

Neither will solve your problem if you cannot connect.
Sounds like a bad or going bad modem/router.