Neither the 10 year old 3521 nor the newer Inspiron 15 3521 (from 2021 or so) support video out over USB, with or without a USB-A to -C connector. You can connect one monitor to the HDMI port on the notebook, but you can't connect the dock to it -- the HDMI is output only. The limiting factor here is the notebook, not the docking station.  If you want to run multiple monitors, you need a mid-range or better notebook -- not this entry level one.  
I have a personal Dell Laptop Model 3521 and this does not have a USB-C port to connect to my ThinkPad docking station. I was told that a USB Type A would not work so I bought a USB-C to HDMI adapter and plugged the USB-C into my docking station and the HDMI into my laptop. I have two external monitors that are also connected to the docking station, but my laptop still will not detect any additional monitors. I have read tutorials and watched video's and almost all of the video's have to do with connecting laptops that have a USB-C port into a docking station, but not connecting HDMI to a docking station with an HDMI adapter. Can someone please help and explain if I am missing something? Thank you
Help with connecting Inspiron Model 3521 to multiple monitors using ThinkPad 
Dock Station