How to connect regular iPad (6th Gen) to external monitor?
This is an excellent question as converting in this direction (Lighting/HDMI to DisplayPort) very rarely comes up.  Offhand, I don't know of any Lightning to DisplayPort adapters and I don't suspect that you'll see any as the market for that type of adapter is very small.
So, to do this, you will need the following:

The Apple Lightning to Digital AV Adapter (HDMI output)
An active (powered) HDMI to DisplayPort Converter
A USB power source.  Either on the monitor or via a USB power adapter plugged into the wall.  A regular phone charger (12W) adapter is more than sufficient.
The HDMI and DisplayPort cables to make the connections

Why is this so convoluted?  Unfortunately, going from HDMI to DisplayPort isn't very easy.  There are HDMI monitors and televisions everywhere, so manufacturers build for that.  DisplayPort monitors are designed for use with higher end computers (like Macs) and not for &quot;consumer grade&quot; displays.

Display converters are one way, meaning the descriptor tells you which direction the conversion goes (i.e. &quot;DisplayPort to HDMI&quot; or &quot;HDMI to VGA&quot;).
DisplayPort is most prevalent on high end computers (like Mac) whereas HDMI is ubiquitous.
It's easier to convert DisplayPort to HDMI.  As such, you can get both active and passive adapters.  Going from HDMI to DP,however is not as easy so an active adapter is required.
Because it's active, power is required, thus the USB-A connector on these adapters.  If the monitor has downstream USB ports, it will be able to supply the necessary power.  If not, a simple USB phone charger that plugs into the wall will do
DisplayPort has power on it's bus, so it can &quot;self power&quot; an active adapter.  HDMI doesn't, this is why it needs additional power.

An alternate solution would be to connect a Mac to that monitor and Airplay your video to it.