Not detecting external displays

The Mac generally supports one display to a cable.

The one exception is when using a genuine Thunderbolt device (Dock or Display) on a genuine Thunderbolt cable, then up to TWO displays can be supported. USB-only is not supported.

Apple does not provide support for DisplayLink "fake" displays. You would have to get support from the Dock maker or DisplayLink for that.

just what I wrote above.

One display to a cable out of the Mac, except for Thunderbolt. No dock can ADD the capability to support a fully hardware accelerated display if the Mac is not sending out that data.

Most Docks are not produced to connect to a Mac -- they are for that other computer.

The Apple standard is that its built-in hardware-accelerated displays are suitable for production/display of cinema-quality video with NO dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or other issues.

Because of this requirement for high quality, the cable needs a lot of bandwidth in MANY cases, especially approaching 4K in size. Rather than support some multiple displays on a cable and have Users left calculate whether there is or should be enough bandwidth to support their exact setup, Apple has opted not to support multiple displays on a cable, except for Thunderbolt.

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You can add as many "fake" displays using tricks like DisplayLink stunt-boxes, as you like. But these are not good enough for full-motion Video, only slow to change data like stock quotes, program listings, or spreadsheets.