Apparently there's ways and means. It's much harder to do, you have to ask a British court or go via the Hague, but not impossible.
This from the guardian a few years ago:
If Prince Andrew were lawfully served with a subpoena – be it for a grand jury, or a trial or deposition – generally speaking, he will need to comply.
However, it is far more difficult to serve him outside the US.
“Due to the heightened measures of security that surround the royal family, it certainly is far more difficult to walk up to a member of the royal family and serve them a subpoena as you would a private citizen,” Weinstein said.
This from BritBrief.co.uk (I have no idea how credible a source they are)
In a significant development that reignites the transatlantic legal firestorm surrounding the British royal family, former US Attorney General William Barr has confirmed that Prince Andrew remains a person of interest in the sprawling Jeffrey Epstein investigation.
The Duke of York could yet be subpoenaed and compelled to provide testimony to American prosecutors, Barr revealed, shedding new light on the Department of Justice's persistent efforts to unravel the full extent of Epstein's network
I might be getting legal speaking confused. He was previously served papers by Giuffre's lawyers several times and evaded them on each occasion.