While most folks would recommend a modified thinset mortar for your application, as Dana is hinting I believe, a whole lotta stone was set successfully for a whole lotta years before the advent of modified thinset mortar. Before the advent of any
thinset mortar, matter of fact.
If your previous tile surface was properly prepared, and if mortar was properly keyed into both surfaces, your new tiles should be adequately bonded.
Now, that's not to say there might not be some structural issues at play, here, but that mortar shouldn't be a big problem.
Ah, now I see Dana has added some clarification to his post and his concern was as I expected. While Premium Plus may not have been my first choice of dry-set mortar, I'm still thinking it'll do OK if it was properly installed.
Do tell us how the old surface was prepared and how the mortar was applied.
And the 24 hours should be an adequate wait before grouting.
My opinion; worth price charged.