Unable to use external monitor's full resolution using Macbook Pro
Dude, i can tell you from personal experience; after buying four different usb-c hdmi adapters that the ONLY adapter that enabled me to utilize my external monitor full resolution + high refresh rate without having to hack internal /System/Library code or messing with SwitchResX/other b.s. GUI garbage;
is the belkin usb-c to hdmi adapter (the white version, do not use the old-black version -- its got terrible refresh rates). I also have apple's usb-c dongle (hdmi,usb-c,usb-a)[but it's utter garbage...] (didnt work); neither did the belkin black; nor did some other 'KANEX'-brand crap that i stumbled upon while cruising down a local shopping-mall. MBP 15" latest model i9's and what not (all the bells and whistles...); external monitor is an LG-4K 27" (supports real high res).
hope it helps

https://www.belkin.com/uk/p/P-F2CU038bt/
This one doesn't give you refresh rates higher then 30Hz(despite what it says on the website)

this one does : 
https://www.apple.com/uk/shop/product/HKQ22ZM/A/belkin-usb-c-to-hdmi-adapter