Running openWRT on an older RPi2 with 2 WiFi dongles as a travel router. However trougput speed is very slow (< 2mBit/s). I'm not expecting an RPi2 to be blazing fast, however <2 mBit/s seems too slow even for this Pi
1 dongle connects to (public) wifi.
1 dongle generated a new WiFi SSID to connect to
config interface 'loopback'
option device 'lo'
option proto 'static'
option ipaddr '127.0.0.1'
option netmask '255.0.0.0'
config globals 'globals'
option ula_prefix 'fd36:7f65:eca9::/48'
option packet_steering '1'
option name 'br-lan'
option type 'bridge'
list ports 'eth0'
config interface 'lan'
option device 'br-lan'
option proto 'static'
option ipaddr '10.6.6.1'
option netmask '255.255.255.0'
option ip6assign '60'
config interface 'wwan'
option proto 'dhcp'
option peerdns '0'
option dns '18.104.22.168 22.214.171.124'
Bus 001 Device 004: ID 7392:7822 Realtek 802.11n WLAN Adapter
Bus 001 Device 005: ID 7392:7822 Realtek 802.11n WLAN Adapter
Bus 001 Device 003: ID 0424:ec00
Bus 001 Device 002: ID 0424:9514
Bus 001 Device 001: ID 1d6b:0002 Linux 5.10.161 dwc_otg_hcd DWC OTG Controller
Per @slh's point, you can buy a purpose built travel router for relatively cheap. Many of them are supported by OpenWrt, so you can install the official OpenWrt and learn on that... including getting various USB wifi sticks up and running if you really want (although the built-in wifi will probably be better, but for the learning experience, go for it).
I know what my 'buy' options are. Yet i have an old RPi 2 lying around and have relatively easy access to different USB dongles. So i'll be more than happy to continue my quest by using a 'good AP' adapter.
By all means. As long as there is benefit to you (in terms of learning, a project you're excited about, etc.), keep going. But know that with respect to "good AP" USB adapters, there really aren't many in this category. Almost all of them are designed with the primary intent as a STA mode device (some don't support AP mode at all), so they aren't optimized for this use. And the radio architecture (often 1x1) and antenna design (usually small PCB traces, sometimes external) is not going to yield great performance, especially with multiple STA mode devices connecting to it.
@frollic has proved a useful link, but if you find others that are interesting, please post here for the benefit of future readers. And, in that same vein, report back on your efforts -- both successes and failures, so that we can have more info on the forum.
Two radios on the same band in close proximity will jam each other even if set to different channels (and here you have not even set them to different channels). It is likely to work better using one radio as AP and STA combined, if the driver supports that. With a single radio time synchronization is better.
Don't try to use HT40 on 2 GHz unless there are absolutely no neighbors. Your two radios count as neighbors.
Start with a speed test wireless-wired. About 30-40 Mb net throughput is the best that a 1x1 radio on 2 GHz can do. A repeater scenario will lead to half of that.
A worthwhile experiment would be to use a USB-ethernet adapter so you have 2 ethernet interfaces. Use one as a "lan" and the other as a "wan" (or second lan) and then run iPerf between them (using computers on either side). This will give you an idea of your maximum routing speed (remember, also limited by the 100Mbps ethernet and USB 2.0 speeds, but I don't even know if the CPU is powerful enough to saturate that in a routing context).