ObjectUnionOf is not in the OWL EL profile , so it is unlikely that
this feature will be added to ELK any time soon.
You can try to use the ConDOR reasoner , which supports
ObjectUnionOf (not sure about DataHasValue). But ConDOR is in a very
preliminary state, in particular it works only from the command line.
Alternatively, you can use MORe  which combines ELK with HermiT,
and can reason over more expressive ontologies than ELK with some
graceful performance degradation (depending on how many features you
have not in the OWL EL profile).
It would also be useful if you could share with us your ontology, so
that we can add it to our benchmark suit and this way let other
reasoners be optimized for that.
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