Hi Denis,
Unfortunately, Faststone requires the Extensions which are downloadable from Microsoft and that shop is not responding.
I have found "HEIC to Jpeg" which does a good job but will only do a maximum of three HEIC files in the free version. The Pro version does batch conversions but again my problem is that, for some reason, the Microsoft Shop is not responding and I cannot purchase it.
DG
P.S. I have changed my phone from iPhone to Samsung and the HEIC files are from the iPhone. I have made sure that the HEIC option is turned OFF on the Samsung!!