Please tell me, are all USB serial adapters universal？
Nope, there are deffierent , chip, voltage, connectors are not the same.
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While what @M10 said is true, it's fairly easy to find compatible ones.
Most consumer router devices use 3.3V TTL serial UART connections. Many of the adapters you'll find can support multiple voltages -- selected by a jumper or a solder blob or similar.
Do not buy an RS232 or RS485 adapter, as those won't work (due to the voltages involved). You want a standard TTL (usually 3.3V) adapter.
The FTDI chipsets tend to be the most 'universal' in terms of operating system support, but there are many similar USB-serial chipsets that will work on all the major OS's. Just beware that there may be some compatibility issues with devices that use counterfeit FTDI chips.
So. while they aren't entirely universal, these serial USB adapters are quite standard and it should be easy to buy a compatible one. Feel free to ask here based on a link to the device you're looking at as well as what it will be connected to (i.e. what is the brand and model of the router).
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Hi
one of most notable drawback of cheap USB/TTL adapter is that debug LEDs (rx/tx) are driven directly from communication pins. Sadly, but manufacturer forget to put 1 cent worth transistors to drive these LEDs 
this will introduce constant resistance to GND and some routers refuse to boot if RX and/or TX pins are pulled hard enough to GND on boot time
so, be prepared to use scissor 
or similar cutting tools to destroy these LEDs
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The one showing in picture looks identical to those Raspberry Pi debug probe? Which is Silicon Labs CP210x USB-to-Serial, with kmod-usb-serial-cp210x should work under OpenWrt.
Openwrt compatibility is only relevant if the usb side will be connected to the openwrt device. In the majority of cases, the serial ttl connection is on the openwrt side and usb is connected to a computer.
Here's a summary about most available options https://wiki.freebsd.org/USB/Peripherals/Serial and differences
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The picture is taken from the introduction to the USB serial adapter in the official OpenWRT document. The DL-WRX36 I have has become bricked after using OpenWRT. The situation is the same as this guy's. I am a newbie in this area.
FTDI USB2serial adapters are great, if they are genuined FTDI chips - sadly many cheap offers use fake chips and are rejected by FTDI's windows drivers (linux usually doesn't care too much). The other alternatives, e.g. cp2102, ch340g and similar adapters are easier when it comes to driver support. The important aspect, unless explicitly mentioned otherwise (some newer devices require 1.8V), is the 3.3V logic level.
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In that case you con't even need an USB adapter for recovery.
Just just have the correctly named initramfs image on the USB memory stick (as old, small and simple as possible) and you you boot in to the recovery initramfs image, and from SSH console use the fw_setenv command to correct the openwrtboot item that you now have mangled from @S7venLights advice.
fw_setenv openwrtboot 'setenv bootargs console=ttyMSM0,115200n8 ubi.mtd=rootfs rootfstype=squashfs rootwait; ubi part fs; ubi read 0x44000000 kernel; bootm 0x44000000#config@rt5010w-d350-rev0'
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Ah I thought OP wants to use OpenWrt as host to connect some other serial devices.
So sorry about that 
The weird error was corrected. See the original thread for more context.
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