Tile adhesive

[kiloran]

About 20 years ago, we had a sunlounge built. It's on a concrete raft about 6m x 5m and it's tiled inside and out with the same tiles, with the outside tiles occupying a band about 1.4m wide on two sides.

We have a problem with the external tiles becoming loose over a few years, presumably due to sun, frost and rain. You can see some of the gaps here:
https://imgur.com/WfXPvVi
The internal tiles are fine.

I initially reset some tiles using a Unibond adhesive, which was not very good. About 6 years ago I chatted to the local tile specialists and they recommended BAL Rapidset https://www.bal-adhesives.com/products/rapidset/ but I now find that this is suffering the same problem. I also used a BAL grout https://www.bal-adhesives.com/products/bal-grout/

Any recommendations for a good external tile adhesive which will give a longer effective life?

--kiloran