ADUM5028-5BRIZ inductivity problem
This is a very capable and unusual DC to DC converter.
Operation frequency is set internally to 180 MHz (!).
Power is transferred via an internal air cored transformer.
The inductances mentioned in table 20 (as ScienceGeyser indicated) are ferrite beads on the Vss and Ground leads. Their function is to isolate IC generated noise from the power lines. The beads play no part in the frequency determination.
They are specified to have an IMPEDANCE of &gt;= 1.8 kohms across the frequency range 100 MHz to 1 GHz.
Manufacturers of appropriate ferrite beads will specify impedance with frequency in their data sheets.
Two examples of suitable beads are listed in table 20
Manufacturer ............ Part No.
Taiyo Yuden  ............. BKH1005LM182-T
Murata Electronics  ... BLM15HD182SN1
By examining datasheets for these products you should be able to find equivalent products if these are unavailable to you.
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Digikey
BLM15HD182SN1D - 20 cents in 1's.
BKH1005LM182-T 10 cents in 1's
ADUM5020 5V version. $7.67 in 1's.
Datasheet
Evaluation boards and application notes here

Not quite to spec but reasonable
Taiyo Yuden FBMH3225HM202NT
from here