OP, get a mDP cable from a reputable manufacturer like Belkin/StarTech/Accell, they're not expensive and those makers are listed on the DP product portal. Look for one that specs out 5.4 (HBR2), or 8.1 Gbit/s (HBR3) per lane (21.6, or 32.4 Gbit/s for a full 4-lane link) - they should also state that they are DP 1.2/1.2a compliant. Don't buy the "compatible" products. They're much less costly than TB 1/2 cables.
Regarding your issue, TB 1/2 cables are compliant with DP 1.1 only, many newer SST displays require DP 1.2 or newer to function (including all of the Dell, Eizo, and BenQ displays in my offices and home). I've assisted members solve this very issue in these forums...