Wires in Ethernet cable are typically very small, like 24, 28 AWG, so the resistance is going to be very high. You might not get voltage drop over 29 ft but I would still switch to something bigger. Second, just use it without the resistor and see if it works. Its purpose is to dampen reflections so if that isn't happening it's fine; it's more likely in a multidrop configuration rather than a peer-to-peer setup like how it sounds. Also, you need the resistor on both ends, not just one -- if you remove it, remove it from both ends. Third, RS-485 is typically two wires (half-duplex) but you sometimes see 4-wire setups (full-duplex). From what you're saying it sounds like two wires, but if it isn't both Tx and Rx pairs require termination.