I'm currently using an Archer C7, which runs completely fine. I only need around 100Mbps of WAN-Bandwidth and there is not mich LAN traffic, so performance-wise I am perfectly happy.
Unfortunately, in the room furthest from the router WiFi reception becomes pretty bad, at least on my main work laptop. My phone and other devices manage well enough. The main problem is that the 2.4Ghz band is totally congested, with each neighbor having a shitty router and then some shitty access points as well. Most of the time I cannot even connect due to what I believe are dropped packages: I get timeouts and need to try multiple times. Despite these problems, there is a very good signal strength, which makes sense, as it's only a few meters.
I think my solution to this problem is to try and get full coverage using only 5Ghz and escape this madness of channel-jumping neighbor devices. Unfortunately the Archer C7's 5Ghz WiFi barely reaches the aforementioned room at very low signal strength.
So I am looking for a device with a range that is better than the C7, and which might enable me to switch primarily to 5GHz.
I would be glad for any recommendations and comments.
Disclaimer: I don't have personal experiences with the archer c7.
Range is a relatively fixed term among most consumer devices (due to regulation and the 'common' devices and their antennas being involved), in general you can't expect this to change significantly with different hardware. Obviously different devices still have different characteristics which also influence range, but unless your current device is really at the lower end of the spectrum (which it probably isn't, as many users have been quite happy with it so far), you shouldn't expect a new one to make (much of) a difference.
Newer devices (802.11ax) are improving, but not really by extending the total range, but by bringing more throughput over the (more or less same) range. Changing the placement of your current router is probably more efficient than buying a new one, sometimes even small changes (within the reach of your patch cables) can already help. If it doesn't, you should consider if there's a way to (preferably) add an AP somewhere half way to the affected room(s), another alternative could be adding a repeater with a wireless backhaul (always worse than wired options, but maybe good enough; you (almost-) always want to use the 5 GHz for the wireless backhaul).
As a rule of thumb, if you are out of range of reasonable (performance and stability) 5 GHz reception, you simply are too far away - changing routers wouldn't help you in that situation, only additional APs/ repeaters could.
If you still want to go shopping, modern 802.11ax based routers often come with internal antennas - those don't need to be worse than external antennas (wifi 6 routers come with 4-16 and more antennas, hard to mount them externally in a reasonable way); some of my strongest devices have internal antennas. You just can't distinguish that by looking at them (respectively internal- vs. external antennas).
If you have the ability to run a cable to that room you could set up an access point.
For example I picked up two of these from eBay for less than $20 each.
They are great but you do need to have a special USB to CAT5 shaped serial cable to get OpenWRT installed on it. But otherwise it's a really powerful cheap router disguised as a corporate access point.
Running a cable is theoretically possible, albeit inconvenient, because I would have to go through multiple rooms/walls. I was trying to avoid it.
TBH, if I went to the effort of running cable to that room, I would directly use that connection instead.
I really would have hoped to get WiFi reaching 8-10m with a bit of wall/wood in the way.
I used an Archer C7 years ago (Wave 1 802.11ac), and I found the Linksys EA8500 (Wave 2) performed much better on 5GHz. I never had need to do range testing though (I have an AP on each floor of the house). Still, if you try an inexpensive used 802.11ac Wave 2 device and it doesn't help by itself, you could next try it with your C7 as a repeater or in a batman-adv mesh.
However, unless it is a used incredible bargain on ebay, I would get something that supports 802.11ax if you buy another device.
2 or 3 packs of the Zyxel Multy WSM20 (M1) seem to have a good following on the forum - inexpensive outside the U.S. If you find a good buy, a set of those would be good for setting up a mesh network in your home to extend your range. They do not have USB ports, if that is important to you.
Out of the 5GHz 802.11ax devices I've tried, the Reyee RG E5 has the best range (but you'll need to open the case and connect some wires to flash it, and it only has 16MB of flash), followed by the Belkin RT3200. Those two are now my home AP's - one on each floor. Others on the forum have noted the Netgear WAX206 performed well at long range.
I do not recommend the DL-WRX36 - it had the worst 5GHz range and stability of any device I tried and now sits in a storage closet as a spare.
Internal antenna devices with range as good as external antenna devices have been the exception in my experience. They exist depending on the devices being compared, and many are not far behind external antenna devices, but if you are trying to fix this with a single device, I would stick with external antenna models.
Higher dBm antennas broadcast in a flat "donut" pattern around the vertical antenna, which gives better range across a single elevation (e.g., a single floor of a house), but not as good above or below the device on other floors. Lower dBm antennas broadcast in more of a sphere pattern, which can be great for a single AP in the middle of a multi-story home, but the transmit power is diluted over a larger volume, which is not helpful for long range.
My only suggestion is to move the Wi-Fi up high. I got better spread in my house when I put it on top of a bookshelf. Same for your computer get an external Wi-Fi antenna or adapter and move it around the room until you find a better signal.
The other thing (that legally speaking you shouldn't do) is use an "off limits" channel for your country.
Just chipping here to give a rough explainer: lowering TX mean lowering range, but not necessarily the range beyond your own home, therefore reducing the chances of your signal and other's colliding and cause interference. It's about reducing collision rates. You'll find better/more detailed explanation elsewhere, like on OWrt wiki, IIRC. Though from your description of the congestion, maybe it won't help much, but won't hurt to try at least.
You don't need to go all the way, roughly the midpoint from current AP location to the room will do great.