Cheap cable or defective scope? That can’t be right.
You're quite right, a cheap unknown cable gives misleading results......lower than for good cables !
1 GHz rated BNC's and 6 GHz N type with adapters also used.
How about testing the same setup on your SDS5000X?
Frequency and levels values checked against
SDS5054X SDS5104X for three yes three sine wave generators each capable of 200+ MHz, SDG6022X, SSG3021X and an old HP8654B.
However all results ~
+ 5% on 185 MHz for the SDS2104X Plus -3dB point which is darn impressive for a 100 MHz DSO. However until others confirm these measurements all you have is my report, take it or leave it.
I presume all bandwidth filters are off?
Interestly with the 200 MHz BW filter ON in the SDS5054X the result was quite similar which IMO adds to increased confidence in the measurements obtained.
YMMV