The forest certainly does not make it easier, but if there is no signal outside, it should not be inside, because the walls and windows always suppress something, and the antennas should still have some net gain. So at least this 10dB level difference should be there. But there isn't... there's just some statistical scatter.
At least strange, but not such strangeness the world has ever seen.
One more thing - I flew through the thread, but I didn't see the basic and obvious test - if you disconnect the external antennas on a working router with forced external antennas (or unscrew it, e.g. by 90 degrees to the side) then whether the signal levels drop to zero (no indications on Hmanager and no green lines on the router indicator)?