(disculpeu-me per escriure en english…)
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Openwrt version used: OpenWrt 23.05.0
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DISCLAIMER: once I installed openwrt on the tp-link WR6400 v5, i haven’t been able to reinstall the original TP-link firmware!
I spent few hours to get a working connection via SIM card, and did try few configuration, before I had a working LTE connection. Being openwrt, it is proving to archeive a much more reliable LTE connection then the TP-link firmware.
This router is relatively cheap (~60.-e) but it has very little available space on /dev/mtdblock4, and uses the LTE modem in qmi mode, which is a proprietary protocol by Qualcomm 
, documented at https://openwrt.org/docs/guide-user/network/wan/wwan/ltedongle.
I found that only two extra packages are needed:
qmi-utils luci-proto-qmi
In order to install the qmi-utils package, I had to rebuild the openwrt image, gaining some useful space, but dropping ipv6 support.
I used the image builder, as documented at https://openwrt.org/docs/guide-user/additional-software/imagebuilder.%C2%A0 with the following packages removed and added once compiling with make:
PACKAGES=“uhttpd uhttpd-mod-ubus libiwinfo-lua luci-base luci-app-firewall luci-mod-admin-full luci-theme-bootstrap -ppp -ppp-mod-pppoe -ip6tables -odhcp6c -kmod-ipv6 -kmod-ip6tables -odhcpd-ipv6only qmi-utils luci-proto-qmi”
I installed openwrt on the device, following the instructions at https://openwrt.org/toh/tp-link/tl-mr6400_v5
#serve tftp-recovery image as tp_recovery.bin on 192.168.0.225/24
dnsmasq --port=0 --enable-tftp --tftp-root=/path/to/firmware/directory --tftp-no-blocksize --user=root --group=root
# check tftpd is running
netstat -lunp | grep 69
# when powering on router, with reset button pressed, as soon as possible
ifconfig eth0 192.168.0.225 up
Somehow the tftp image I built was not working, so I first installed the last official openwrt tftp .bin version, and then I did a sysupgrade with the openwrt sysupgrade .bin image I built.
Once succesfuly installed, I removed eth0.2 from the wan interface, and assigned the qmi interface as wan in /etc/config/network:
config interface 'wan'
option proto 'qmi'
option device '/dev/cdc-wdm0'
option apn 'YOUR APN NAME'
option pincode '****' # your pin code numbers
option auth 'none'
option pdptype 'ipv4'
option delegate '0'
option dhcp '0'
(I successfully tested this configuration with digi and movistar APNs)
Once rebooted, it takes a minute for the qmi interface to connect to the LTE network and obtain an ipv4 address.
You can check the LTE signal strenght (and a lot of other stuff) with uqmi:
root@OpenWrt:~# uqmi -d /dev/cdc-wdm0 --get-signal-info
{
"type": "lte",
"rssi": -71,
"rsrq": -15,
"rsrp": -108,
"snr": 1.400000
}
You can check the status of the network connection on the interface section in Luci or with ifconfig on the terminal:
root@OpenWrt:~# ifconfig
wwan0 Link encap:UNSPEC HWaddr 00-00-00-00-00-00-00-00-00-00-00-00-00-00-00-00
inet addr:10.19.10.145 P-t-P:10.19.10.145 Mask:255.255.255.252
inet6 addr: fe80::745c:fd7e:9bb7:78a4/64 Scope:Link
UP POINTOPOINT RUNNING NOARP MULTICAST MTU:1500 Metric:1
RX packets:893508 errors:0 dropped:0 overruns:0 frame:0
TX packets:447789 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1256641567 (1.1 GiB) TX bytes:22103352 (21.0 MiB)
Finally, I wanted to have the “internet” led blinking on the router, once packages are received from an LTE connection. I added this section in /etc/config/system:
config led 'led_wan'
option 'name' 'wan'
option 'sysfs' 'white:wan'
option 'trigger' 'netdev'
option 'dev' 'wwan0'
option 'mode' 'rx'
Next thing to do is to write a script assigning the LTE signal strength to the three “signal” leds: white: signal1 white:signal2 white:signal3
I am still able to receive and read SMS messages, via uqmi commands on the terminal:
uqmi -d /dev/cdc-wdm0 --list-messages --storage me
uqmi -d /dev/cdc-wdm0 --get-message 0 --storage me