According to this Website on LTTPR it isn't the GPU card or source that produces the LTTPR signal rather it is a dock with LTTPR that you connect to the GPU card Display Port and to the Monitor: Introduction to DisplayPort LTTPR
Here is one Dock that support LTTPR from above link:
It seems as long as the GPU Card has DP version 1.4 it should work. So you need to find a AMD GPU card with an output of DP 1.4 and then connect it to something similar from above website.