The hope is that the bootloader has already a function for recovery that kicks in on its own, and you just need to set up a tftp server, and set yout PC to a specific IP address for recovery, but I don't know that device, nor what yuncore does usually, so it would be best to see that first.
Only info I could find is another yuncore product here https://wiki.openwrt.org/toh/yuncore/cpe-880 it seems to be pretty default, and it has TTL pins soldered already.
It is a bit time-consuming, especially if it is your first experience with serial and bootloaders, so be warned.
this is a usb-ttl adapter I also use (not affiliated with that seller, it's just one of many selling the same stuff).
TTL pins are usually 4, a ground, a Tx (transmit) a Rx (receive) and a power pin. If you are lucky they are already soldered or there are clear pin holes you can solder pins on, if you are not, they are accessible from contact pads. this is an example of device with obvious pin holes for TTL.
Find out what of the pins is the power pin with a multimeter, and DO NOT use it (the usb-ttl adapter is powered by USB anyway), the other 3 pins you can simply find out by trying them (as there is no power involved, mistakes don't damage the board).
From the PC, the usb-ttl adapter is seen as a serial adapter (because it is a serial adapter after all), and so you should use putty/kitty or other programs set to talk to serial, most common settings (also used in the other yuncore device above) are
Bits per second: 115200
Data bits: 8
Stop bits: 1
Flow control: None
Then you should see text scrolling by when you power up (see the "bootloader boot log" in the entry about the other yuncore device for an example).
U-boot (the bootloader) usually writes "hit any key to stop autoboot" and in this time you should write something to stop boot sequence. Some devices require a specific word (tp-link wants "tpl") but for many u-boots any keyboard key will work.
Once you pressed a key to stop the boot, you are in the bootloader's console, assuming that the u-boot wasn't modified to show a custom menu (on some devices it will say "press 1 to do X, press 2 to do Y), write "help" to get a list of commands.
If it has "tftp" or "tftpboot" in the list, you can recover it manually by tftp.
"printenv" to see what are its settings, it should show a "serverip" and "ipaddr" variables with IPs in them.
serverip is the IP you must place the tftp server at so you can transfer the firmware over, ipaddr is the ip address of the device itself.
you can usually change them (temporarily) by writing "setenv ipaddr write-new-address-here", note that basic networking still applies. The devices must be in the same subnet, so having these two IPs set up as 192.168.1.1 and 192.168.1.2 will work, while if yous et them as 192.168.2.1 and 192.168.1.2 will not work
If you got to this point, you can install a tftp server in your PC, there are plenty of tutorials, it's usually very easy as tftp is a very simple protocol used by bootloaders and other very simple systems.
Set your PC to a fixed address specified in "serverip" and connect the ethernet cable to the device.
The command to transfer the file is either tftp 0x81000000 code.bin or tftpboot 0x81000000 code.bin
When transfer went well, you must erase the flash partition with firmware and write a new firmware image to that flash partition.
I don't know how that device is set up in LEDE so I'm not going to post the erase/write commands that could screw it up even further (there are partitions with hardware wifi calibration data, if these are erased the wifi will not work anymore), see if you can contact the guy that made that patch I linked above (there is an email in the commit description) to get this information.