Second monitor (HDMI) stuck on 800x600 resolution when connecting via thunderbolt DisplayPort

[AaronGU51]

Hello everyone,

I have an HP Elitebook laptop that I use with the HP thunderbolt docking station.   The docking station has one HDMI and 2 DP ports. I have also have two Samsung HDMI monitors that I want to connect to. I connected the first monitor into the HDMI port and it works great with the optimal resolution.  The second monitor has an HDMI port but since the docking station only has a DP port, I presumed I have to get an adaptor.

I bought an DP->HDMI adaptor (https://www.amazon.co.uk/gp/product/B017Q8ZVWK/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1).  However, when I connected it, the second monitor is frozen into a 800x600 config and it does not let me modify it. I tried updating the drivers, restarting, reconnecting, and nothing helped.  On reading some help forums like this (https://h30434.www3.hp.com/t5/Desktop-Operating-Systems-and-Recovery/Stuck-in-low-resolution-640x480...), I am concluding it is due to the cable adaptor I am using.  

I wanted to check a few things:

1. I wanted to check with you on whether this DP->HDMI adaptor on the HP store (https://store.hp.com/UKStore/Merch/Product.aspx?id=F3W43AA&opt=&sel=ACC#reviews)  would be recommended by HP for this purpose, or if it would create the same problem?
2. Because the docking station has a DP and my monitor is HDMI, am I correct in getting a DP->HDMI convertor, or do I need to get a HDMI->DP convertor?  I have searched so many forums and I can't seem to get a clear answer anywhere.

Thanks in advance for your patience and help.

[AaronGU51]

As an update, I have tried replacing the HDMI cable as well as the adaptor and these are definitely not the problem.  The issue still persists.  Any help would be recommended.