I know this has been a while but I thought I'd respond if others happen upon this like I have.
"I bought an USB-C Digital AV Multiport Adapter but did not notice that Dell2214H only has DisplayPort."
I followed the link given to the display specs and I see it has DVI-D, connecting DVI-D to HDMI takes only a cheap passive cable to work and does not require any protocol conversion. Most DP to HDMI adapters are passive, meaning they require a DP output with backward compatibility to HDMI and DVI to work. These are called DP++ ports and will have a DP++ symbol to mark them. Active adapters only work one way, as in if it's made to adapt DP to HDMI then it cannot adapt HDMI to DP. It's likely you have a passive adapter or an active adapter plugged in backwards.
"There is no USB-C to DisplayPort in Apple site, maybe MBP 2017 cannot output to DisplayPort?"
DisplayPort is the only thing it can output. Apple sells an adapter to HDMI because there is no adapter needed to get DP video out.
"What should I do next to make my Dell display work?"
Get a passive cable with HDMI on one end and DVI-D on the other, use the DVI input instead of the DP input.
HDMI is little more than DVI with a different connector, it's more than that but for the most part DVI equals HDMI and HDMI equals DVI. Think of it like getting a little dongle to plug a mouse into a USB-C port, or a 1/8" headphone to stereo RCA jack cable. It's the same thing, don't make it complicated.
If you want to plug a MacBook with USB-C into a DisplayPort display then just get a simple passive USB-C to DP cable. If you need more ports then get any of a number of cheap mini-docks so you have DP, USB, and maybe USB-C power pass-thru.
If there is a situation where the only output you have is HDMI and the only input you have is DP then only an active HDMI to DP adapter will do. A passive cable will make a physical connection, but no video will pass through. Passive adapters only work from DP++ to HDMI or DVI. Active adapters will work too but unless there is a concern with needing compatibility with DP (not DP++) outputs, or to get higher resolutions from HDMI 2.0 (which is not part of the DP++ backward compatibility), they are not gaining anything for the extra cost.