May 2023

How to use a phone as an uplink to your existing wifi router

We just came out of a 5 day power outage following an ice-storm with so much freezing rain that many tree branches collapsed.
Internet was not affect until about 20 hours into the problem. Bell Canada made no efforts to bring generators in order to keep phone lines and dsl connections running. The curb side DSLAM equipment has a battery buffer but the equipment goes dark once those batteries run out. I decided to use a cell phone as an up-link to my existing home wifi router. Here is how to do it.

Bell Canada, curb site phone and internet DSLAM, battery powered/buffered, those red blocks are batteries

The cell phone network was heavily congested but it worked well off hours (early morning, lunch time and late evening). It seems cell towers had generators. In other words it was possible to get a usable data connection via the cell phone network.

Instead of re-configuring every device in the house I wanted to bring everything back up and use a mobile phone as an up-link to my existing router. The problem is: phones don't have Ethernet plugs. It used to be possible to buy wifi Ap-client boxes (also known as gaming console AP-clients) that have a wifi antenna and out comes an ethernet cable. These were popular more than a decade ago when gaming consoles had ethernet jacks but no wifi cards. Those AP-clients have unfortunately disappeared from the market however the free wifi router software OpenWRT (https://openwrt.org/) still supports this feature.

Steps:

Start the wifi hot-spot on your phone. Note down the SSID (name), password and channel (most android phones use wifi channel 11, if unsure scan the network from a laptop).
Install openwrt.org on some old wifi router. I used a linksys wrt54gl linux router. I installed this version of software: https://downloads.openwrt.org/backfire/10.03.1/brcm-2.4/openwrt-brcm-2.4-squashfs.trx You have to rename the file to openwrt-brcm-2.4-squashfs.bin after downloading it otherwise the stock linksys firmware will refuse the upgrade.
Openwrt initial config: plug ethernet cable into a LAN port, router IP=192.168.1.1, configuration URL http://192.168.1.1/ , user=root, password=--leave empty--
Configure the openwrt router as client (see screenshot further down)
Configure your wifi router to obtain it's public IP via DHCP. This might already be the case if you have a cable modem type of box or a more modern DSL modem.
Run an ethernet cable from the openwrt AP-client (plug it into one of the LAN ports) to the WAN port of your wifi router. Reboot the wifi router to force it to send a DHCP request.

The good thing about this solution is that you can have it pre-configured and ready to go. If there is an internet outage then your just power on the openwrt AP-client and you move the WAN cable on your router to the AP-client.

This setup allows you also to position your cell phone such that it has the best cell network signal. You can place the cell phone somewhere outside the house and still use all the computers and devices in the house.

Network diagram, Cell Phone data connection as up-link to your existing wifi router

Screenshot_2023-04-08_at_12-18-47_OpenWrt_-_Wifi_-_LuCI.png

OpenWrt configured as AP-Client

It's a great backup internet connection and it does not require any re-configuration on your existing devices. The setup is using double NAT-ing (in the wifi router and in the cell phone) but that is fine for all normal applications.

References

https://downloads.openwrt.org/backfire/10.03.1/brcm-2.4/openwrt-brcm-2.4-squashfs.trx, the openwrt I used for my Linksys WRT54GL. The Linksys WRT54GL version 1.1 router is out of production but it is still very popular and you can get it for about 20$ on ebay.

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