Vostro 5490 with which Dock?

@FionaL  Ok, then it sounds like either your dock unit or its power supply is malfunctioning.  If you had another of either, you'd be able to narrow it down and potentially purchase a replacement for only one component (although a standalone WD15 dock unit would have to be purchased through a third party since Dell doesn't sell them without power adapters), but if not and you're considering a whole new dock and power supply, I would recommend the WD19 over the D6000 as long as you don't plan to use a display setup more advanced than dual 1080p.  There are two reasons for this:

• The D6000 only supplies 60W, whereas your system is designed for 65W.  Some earlier Dell systems wouldn't throw a warning about this minor shortfall, but newer ones seem to do so, and seeing a warning at boot time and within Windows about an undersized power source would get old fast.  The WD19 130W version can pass up to 90W through to the attached system.
• Whereas the WD15, WD19, and most other docks tap into native GPU outputs, the D6000 uses "indirect display" technology called DisplayLink (not to be confused with DisplayPort).  That comes with some drawbacks that can be significant in some people's use cases.  I wrote about those in detail in the post marked as the answer in this thread.  The only reason I'd suggest even considering the D6000 in your case would be if you wanted to be able to run a more advanced display setup than dual 1080p.  In that case, the D6000's method of handling video becomes an advantage, because it means it can handle dual displays up to 4K 60 Hz plus a third display at up to 4K 30 Hz.  Your system doesn't offer enough native GPU bandwidth over its regular USB-C port for that, which is why you're limited to dual 1080p when using the WD15 and WD19 -- although again, these docks won't come with the potential drawbacks of DisplayLink.  And your system doesn't have Thunderbolt 3, which would have allowed access to more native GPU bandwidth.  If it did, you could have used a dock like the WD19TB to run dual displays up to 4K 60 Hz or triple QHD displays while still avoiding DisplayLink's drawbacks.