All I saw was the hardboard. Had a similar issue with a wooden bathroom floor, tried a flexible grout but that failed too. Ended up replacing the tiles, and using BAL single part flexible adhesive and then BAL rapid mat.
If you are just going to stick a couple of tiles down, I think I have some bal single part left and a sucker to hold the tile till you get the height right. Have mapei flexible grout in a sandstone colour too.
All I saw was the hardboard. Had a similar issue with a wooden bathroom floor, tried a flexible grout but that failed too. Ended up replacing the tiles, and using BAL single part flexible adhesive and then BAL rapid mat.