External monitor resolution issues

I own:

• iMac, mid 2011, 21.5 inch, updated to macOS Sierra 10.12.6
• MacBookPro, mid 2012, 13 inch, updated to macOS Sierra 10.13.6
• Phillips 220WS, 21 inch monitor, resolution of 1680x1050 at 60Hz

I often use my laptop with the monitor as an external display using a VGA adapter. When I plug it into my laptop it immediately recognises the monitor as the Phillips 220WS and adjusts the resolution appropriately.

When I try and do the same using my iMac, it recognises the monitor but only as a 'VGA Display' and therefore displays my screen at a default resolution of 800x600. When I go into display options and try to change the resolution using the scaled option, it does not give me an option to change to 1680x1050 even when I option click to display the full range of resolutions.

I have tried downloading apps that give me more options for screen resolution such as EasyRes, DisplayMenu and SwitchResX but the former still do not give me the option of 1680x1050 and the latter is not free and will require me to disable the SIP and alter a system file which I am not prepared to do.

Could anyone offer a solution?