3 GHz and woods? Can you say attenuator? Real good attenuator!
You changed the "internals" but not the sign. The working sign is only 15 meters away. Move the non-working sign across to that location and see if it works there. When there are obstacles in the way, even a one wavelength difference in location can mean the difference between a good signal and a bad one. At 3 GHh the wavelength is 0.1 meter or 10 cm or 4 inches. At 15 meters your two signs are 150 wavelengths apart. That is a long, long distance; enough for reception to be completely different. Anyway, move the whole sign, not just the internal circuitry. Connectors and cables and other things do go bad.
After doing that, you will know if it really is the location or just the sign itself. If it is the sign itself, some kind of repairs are needed. If it is the location, look for a big metal object nearby. A water tower would be nice or a big building; the higher the better of course. Aim at that with a directional antenna (Yagi). If there are no obvious reflectors try every direction, even directly opposite from the straight line path - strange things do happen with RF.
Another thing that you can try is the polarization of the antenna. Since those woods are blocking your direct path, you are looking for an indirect one. When a signal is reflected often the polarization can change. Switch between horizontal and vertical polarization and try the two 45 degree positions between them. You do this simply by rotating the Yagi antenna about it's long axis.
Finally, you said it when you said "height". It may not be practical, but the higher you can locate your antenna, the better the signal strength can be. One standard for measuring broadcast signal strength is with an antenna at 10 meters above ground. That gets above most single story buildings and a lot of trees and other plants. I have seen this work in a very dramatic fashion while making field strength studies in VHF and UHF bands. Of course, the same applies for the antenna you are trying to connect with, but you may not have any way to change that. But it is possible that just moving the antenna one or a few wavelengths can have a big effect on the signal strength. When multiple paths are considered, one wavelength (10 cm) or even a half of one wavelength (5 cm) can be a completely different location.
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