I had  this problem a few years now with the 707, tonight I finally fixed it sort of...

Here I am flying, please note the...
NDB's at 367.0 (MO) at bottom and 271 (SC) to the left
VOR's at 114.60 (MOD) at the bottom. and 116.00 (ECA) to the left


I have ADF1 & ADF2 radios tuned to the above freqs.:







And the two NAV radios also tuned to those VORs.



The levers on the RMI are set to VOR and indeed the 2 arrows are pointed directly towards the VOR stations (ECA and MOD).



But, now the levers are reset to ADF and the needles are parked and NOT pointing to the 2 NDB navaids on the map.

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There are two RMI gauges on the panel.
Left side the captains RMI, on the right the copilot's RMI.

The above RMI images are all from the Captian's (left) side.

But now on the right side panel I have modified the panel.cfg file to remove the CaptainSim RMI and replaced it with the Microsoft default DC3 Radio Compass), like this...

[VCockpit01]
size_mm=1024,1024
pixel_size=1024,1024
texture=$FPSP
background_color=0,0,0
gauge00=Captain_Sim.x707.R01!R01_001_light, 625, 824, 56,55
gauge01=Captain_Sim.x707.R02!R02_clock, 50, 300, 155,151
gauge02=Captain_Sim.x707.R03!R03_aplight, 625, 750, 55,55
// gauge03=Captain_Sim.x707.R05!R05_rmi, 275, 775, 200,200
gauge03=Douglas_DC3!radio_compass,  269,760,200,200
gauge04=Captain_Sim.x707.R06!R06_001_light, 500,450, 58,57
gauge05=Captain_Sim.x707.R06!R06_002_light, 498,526, 58,57

Notice the 2 DC-3 gauge needles (ADF1 and ADF2)  are indeed pointing to the 2 NDB navaids on the map (MO AND SC).





I took the time to post these images to demonstrate what I was trying to say:
1) FSX default DC-3 Radio Compass works correctly  with NDB navaids.
2) While the CaptainSim RMI I do not believe does work correctly with NDB, but does with VOR navaids.

But I could be mistaken here (pilot error)