Hmm, I wonder whether using a repeater would even work. When someone tells me then don't even get a signal, that's a problem. As they say, nothing times something is still nothing. IOW, you can't repeat a signal you can't even reach. I realize you're hoping that somewhere in the "midlands" lies a sweet spot, but that's iffy, or may still be very weak, esp. w/ the kind of environment you've described. Has he considered using powerline or MoCA as the bridging mechanism? And if he still needs wireless, use a wireless router/AP at the other end of the powerline/MoCA bridge.
[primary router]<-- wire -->[powerline adapter #1]<-- interior power lines -->[powerline adapter #2]<-- wire -->[wireless router/AP]
But let's put that aside for the moment. After looking at the WNDR3700 manual, it appears to me it uses WDS bridging as its repeating mechanism. That’s another problem. WDS is not a wifi-certified protocol, so implementations vary, and thus incompatibilities are all too common. For all intents and purposes, it's likely only to work between the same manufacturer's products (sometimes it doesn't even work across different product lines from the same manufacturer). That's one of those nasty little details the manufacturer leaves out of their documentation (no one likes to admit they're not using industry standards). You could spend endless hours in frustration getting this to work. It may even appear to work, but work only intermittently, or fail later. Just not a pretty picture UNLESS they are the same device, and thus have the same implementation.
What you might need to do at that point is consider installing dd-wrt (third party firmware), as long as the WNDR3700 is v1 or v2. Then configure the router as a "repeater bridge". That's a universal repeater. It will work w/ ANY wireless router, including the Ubee.http://www.dd-wrt.com
And configuring dd-wrt as a "repeater bridge" is yet another discussion. Not the hardest thing in the world, but given you need to install dd-wrt itself, and configure it for "repeater bridge", it's non-trivial. That's another reason it may be easier to use powerline/MoCA. Those are pretty much plug-n-play solutions.