How can I determine whether persistent VDSL bufferbloat is caused by my modem/router or my ISP?

I am trying to determine whether my latency problems are caused by my ISP or by my VDSL modem/router. Now before you just suggest buying a new expensive thing. That is an investment and I might just spend money to find out it's my ISP all along. However, if it is indeed my ZXHN H168N V3.5 I can save and get better hardware. But I gotta rule that out first if possible. Now note that I'm in Africa and the following numbers are good for my country. 30 Mbps download / 3 Mbps upload package Actual speeds are usually 26-28 Mbps down and 2.5-2.8 Mbps up Idle latency 8-10 ms. I'm fine with that speed.

Question by Seallussus on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.