Old thread, but I've been trying to solve this for years and I only just solved it today after reading some of the other suggestions, so thanks a million to you all especially @thiago37 and @TrueTAdam
I'm on an old mid-2011 iMac running High Sierra.
First of all the app SwitchXRes costs money and only has a 10 day trial, so I skipped that.
Originally I was using a display port cable in the monitor to MiniDisplayPort (Thunderbolt) in the iMac but this didn't work.
I tried a MiniDisplayPort to MiniDisplayPort (Thunderbolt) and it worked - full 3440x1440 on my LG Ultrawide and 2560x1440 on my iMac.
GreenBeanSheen
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