Open the router and look for solder points or pins that say VCC, GND, RX, TX. You connect the TTL cable to the these pins
(picture is just an example)
Configure your favorite terminal with the following settings: 115200 baud, 8 bit, no parity, 1 stop bit, no flow control.
Tp-link Telnet Commands are something like that
Login: admin
Password:
> help
?
help
logout
exit
quit
reboot
adsl
xdslctl
xtm
brctl
cat
loglevel
logdest
virtualserver
ddns
dnsproxy
syslog
echo
ifconfig
ping
ps
pwd
sntp
tftp
wlctl
arp
defaultgateway
dhcpserver
dns
lan
lanhosts
passwd
ppp
restoredefault
route
save
swversion
uptime
cfgupdate
swupdate
exitOnIdle
wan
mcpctl
utility