But was there any plan B on any NDB approach if/when the NDB should fail?
Yes there was Jan, it is called
Very high frequency
Direction
Finding (VDF). The pilot transmits (VHF) requesting either a: QDM = magnetic track to the station, QDR = magnetic track from the station and QTE = true bearing from the station (usually used for chart plotting), triangulating bearings from a number of stations could be made. The VDF let down is when the pilot once over head from the series of QDMs turns onto an outbound track for a given time whilst descending and the track maintained/corrected by obtaining a series of QDRs, then turns inbound on a given timing, continuing to receive bearing from ATC (now QDMs) whilst descending to the procedure minimum height (MDH). This procedure was available for many years after the NDBs became in use so could be a backup. The military used QGH, the difference being that the controller calculated revised heading corrections from the pilot transmissions and also instructed the pilot when to descend in accordance with the procedure. VDF bearings for homing is still available from many ATC aerodrome stations.
The ADF was so named because it dispensed with the controller pilot interaction and therefore was considered 'automatic'. Both methods were usually available at a ATC aerodrome.