@Dottordan 

Hi, Are you able to tell if your phone works on 4G or 4G+?

Actually the TL-MR6400 is a LTE CAT4 product, it doesn't support 4G+, but most smart phones now support higher LTE category such as LTE CAT6, CAT9, CAT11 etc. which are 4G+, so it doesn't make sense to compare two products with different LTE category.  

If you have another LTE CAT4 product to compare with the TL-MR6400, please test the speed again on both of them at same location and same time, then let us know the results.

In addition, please test the MR6400 speed via both wired connection and WiFi connection to see if it is just WiFi issue.