Sometimes I get garbage characters, to fixed it I switch the USB cord.
Make sure if you are in Gnome for example - not pure screen - make sure disable UTF8, choose some ASCII based like Central European etc...
https://www.hiroom2.com/2017/07/02/debian-9-tftpd-hpa-en/
apt install -y tftpd-hpa
systemctl disable tftpd-hpa
service tftpd-hpa start
Set interface:
IP: 192.168.1.33
Netmask: 255.255.255.0
Gateway: 192.168.1.1
Profile name: TTL
The default folder is:/etc/default/tftpd-hpa
which defaults to/var/lib/tftpboot
tftp 192.168.1.33
get openwrt-21.02.5-mvebu-cortexa9-linksys_wrt1900acs-squashfs-factory.img
ufw disable
ls -l /dev/ttyUSB*
apt -y install screen
sudo screen /dev/ttyUSB0 115200,cs8,-ixon,ixoff,-istrip
sudo screen /dev/ttyUSB1 115200,cs8,-ixon,ixoff,-istrip
reboot
setenv ipaddr 192.168.1.1
setenv netmask 255.255.255.0
setenv serverip 192.168.1.33
setenv firmwareName openwrt-21.02.5-mvebu-cortexa9-linksys_wrt1900acs-squashfs-factory.img
setenv firmwareName insomnia-openwrt-21.02.5-mvebu-cortexa9-linksys_wrt1900acs-squashfs-factory.img
run update_both_images
ufw enable
service tftpd-hpa stop