You may continue to your MDA/DH if you have one. In the case of a no DH CAT IIIb approach you have no DH so you cannot continue your approach to it.
Background is that with a DH you can continue to the DH and very well meet the required visual segment and therefore decide to land. With no decision height no decision can be taken, no visual segment is possible and therefore the approach has to be discontinued once the RVR falls below 75m.
In my outfit we operate a mix of CAt IIIa and CAT IIIb capable aircraft of the same type and the regulator therefore demanded that we change the sentence in our OM A as follows:
If, after passing the outer marker or equivalent position the reported RVR/visibility falls below the applicable minimum, but in no case below 75m, the approach may be continued to DA/H or MDA/H.