Hi what router do you have?
What build of OpenWrt?
What band are your devices on?
When using a wifi booster or repeater you will loos some bandwidth because of the router radio has to talk to the repeater and then talk to your devices and back and forth. Using a cable will all ways be faster. You can read more here:
For what it's worth, some of the best RF performance I've had from a desktop router was the Netgear R6100. It's a very plain looking box with internal antennas, but due to PA / LNA chips on both bands, it can belt out some coverage. (Until the 5 GHz radio fails, which happens a lot.)
Any gain in an antenna is produced by directionality.
An omnidirectional is the same in any direction, that Yagi type with the huge +20db is focused like a flashlight and is much WORSE, in every other direction. There are "omnis with gain" with a pattern like a pancake, which have gain on one floor, but maybe are worse to someone a floor up. Think in terms of patterns.
You have to connect an antenna to your radio with a feedline, aka coax cable.
Feedlines have loss, increasing with frequency and length. 2.4ghz and 5.6ghz are pretty high frequencies. The loss is pretty high. With the typical very thin coax, you could loose half your signal strength in 3-5 feet, erasing much of the advantage of your new gain antenna. Avoid or try to use as little as possible.
If you want to repeat a signal, you want to try to not have the repeaters on the same frequency. Wifi only allows 1 station to transmit at the same time. And, if something interrupts, both have to retry. If your repeater is trying to send the same data, best case is half speed, real world goes down fast as collisions, retrys, non related neighbors, etc, use up the airtime. Try to avoid repeating.
Lastly, (or really firstly!) use a wifi scanning tool to check out your neighborhood, to pick the clearest channel, for single access point, or pair for repeating.
Just a slight addition here, there are massive quality (and price-) differences for antennas and feedlines, a half-decent one doesn't come for a few bucks. Furthermore there aren't that many 5 GHz capable antennas on the aftermarket - and especially modern high-end routers often come with quite special antennas (basically two-in-one antennas), it's not that easy to get matching- or better antennas for these.
The only external antenna I've really had to work well is a 24 dB gain 2.4 GHz dish, which could put full bars on a smartphone 500 meters away (until someone parked a bus in the path.) I tried a couple of yagis but they really weren't much better than the omni that came with the device (Picostation M2). TP-Link used to sell those dishes for $50 which was a good deal.
That was a quite special application. Generally I'd buy and use a CPE device as a pre-engineered unit rather than trying to combine an antenna.
Antennas are one of the things you should not buy on ebay or amazon as the sellers will know nothing about them and are therefore likely to sell absolute garbage.
Knowing how these things go, they probably sold you a DVB antenna (i.e. digital TV) or a piece of modern art that looks like one and pulled the dbi number out of thin air.
You need to search for websites that sell only networking equipment if you want to have some reasonable guarantees that you are actually buying an antenna.
I think on eBay or Amazon the mantra of caveat emptor always applies, especially if buying from China based on my experience.
However, they do quote a specification and an expected increase in gain, so I got a refund for the Yagi and was happy with my improvement for the others.
With China one has to look at the stated specs to determine if they are selling Garbage, for example if they quote 12000 mah for a battery cell where the best in the world is 3800 do not be surprised to find the cell full of sand.
Same goes when they advertise a 1Tb USB stick, both of these are what I would call Fake products.
However, I do not think we can tar them all with the same brush, just price the risk into the purchase.
Yes I buy stuff from china all the time and I know my way around the usual obvious scam stuff, but I've never ever had a so bad experience as with antennas. In my experience they are all modern art pieces.
Anything can be a workable 2-3 Dbi antenna and that's the most you are going to get from those kinds of sellers.
That could be a useful approach, but of course, only if your source has real OEM replacement antennas, vs a plastic shell copy and some random wire for the antenna... And of course, how sure can you be, when buying online?
It's tricky and finicky to try to judge signal strengths, as you may know moving an inch or less can cause large changes, or even your body moving near antennas. But, if you get a feel for it, or test with little to no movement, or average a lot of measurements, you can get an idea.
In this case, using your RSSI, you actually saw about the expected gain, so that's encouraging. Hope the tuning (SWR) is correct, and you have a good load for your transmitters, or they will be stressed. Hard to tell without $$ RF test equipment. Since you're seeing the expected gain increase, odds are they are "real" antennas, and should be good loads for the TX's.