According to https://dl.dell.com/topicspdf/latitude-14-5421-laptop_owners-manual_en-us.pdf you have two Thunderbolt USB ports that have Power Delivery. The others aren't.

I'm having the same issue. I have Yoobao 300W power station and when I connect Dell 5421 via USB C cable to USB C 65W PD port and to the thunderbolt port, the laptop charges the power station.

I have a 65W Baseus car charger, and it can charge the laptop using the same cable and same thunderbolt port.

Could not find a setting in the laptop yet, tested by changing battery settings in Dell Power Manager but the issue is same

Most of the dell laptops don't support PD where type c port only transfer data unfortunately. Personally PD should be there for all laptops now a day!

I am using a Latitude 5340 and I also can't charge my laptop with a PowerBank because my laptop charges the PowerBank. 
Why can't there it be an option to select the direction of power. 

I am having the same issue. My Dell 7280 charges fine via USB-C using the travel plug I have.

But not the Baseus 65w power bank. No matter how many times I plug and unplug it.

Aside from that, the laptop discharges even when off. I'll use it out on the train, shut it down and when I boot it up again, the battery is down to 5%.