Thanks for posting. Apologies that your system is not performing as expected.
First I have a few questions:
Does the monitor function correctly when connected to another laptop?
Does the laptop function correctly when connected to another monitor?
What method are you using to connect the monitor to the laptop?
Does the laptop function correctly without an external monitor connected?
Depending on the answers, it may be either a software or hardware related issue. To rule out hardware, please perform a system diagnostics by either pressing F12 at startup or by using Dell Support Assist.
If the laptop hardware checks out, then I would start looking at the monitor, the cable, the connection (at both ends) and the motherboard connection. Next, I would suspect the GPU, and if that's okay, then it could be the monitor.
Here is some information from the Dell knowledge base you may find helpful: