Compact alumina can only be dissolved in a melt. If you find a platinum crucible, you can dissolve Al2O3 for example in a melt of NaOH, Na2CO3 or Na2B4O7, or NaF. Such cooled melts with Al3+ ionen are water-soluble.

From my experience, we needed to dissolve etched alumina on top of aluminum. So we mostly used a mixture of phosphoric acid (6 wt%) and chromic acid (1.8 wt%) at 50 oC. The time depended on the alumina layer (for example 5 μm needed around 3 hours)

Molten salts certainly can dissolve it, such as NaF-AlF3 melts at 800-1000 degrees C. I would think that fuming HF also will do the job.

conc. HF at relatively higher temperature can dissolve alumina after soaking for 2-3 days.

Look up the Bauxite or Bayer-process.

Those are the industrial processes for dissolving alumina.

Another possibility: solid-state reaction at 900 C Al2O3 + NaHSO4 (or Na2S2O7)

Alumina can be dissolved in strong alkali solutions like NaOH or KOH.

For a somewhat related discussion, you may check: https://www.researchgate.net/post/What_acid_usable_of_alumina_leaching_processing