Look up Andrew McNeil on YouTube if you can. He has lots of content and examples. https://www.youtube.com/channel/UCHqwzhcFOsoFFh33Uy8rAgQ
Without a vector network analyzer with the bandwidth for 2.4/5.8 GHz to tune the signal frequency well, you are poking around in the dark when it comes to real performance.
I've built some antennas including a one for WiFi. It made a small difference for awhile, but it turned out there is an error of some kind in the OpenWRT kernel or in the hardware design of the device. It took me a long time to isolate the problem. The router was unable to transition between the 2.4 and 5.8 bands smoothly and was causing problems. I actually get better range and performance by disabling the 2.4 GHz radio entirely. My issue of the bad 2.4 radio is probably unrelated to whatever you're dealing with, but it is something to think about and maybe test out if you're trying to improve router performance.
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