Due to the poor performance of my old POTS ADSL line, I decided to look at 4G Routers. Surprisingly, I could get over 200mbps download and around 50mbps upload!
Compared to my ADSL line of 17mbps down and 1mbps up, this significantly increased speed and performance!
I wanted to make use of my ADSL line, so I turned my Raspberry Pi into an OpenWrt router, and have it monitor both WAN connections.
If it detects the 4G connection is down by performing regular pings to 1.1.1.1, it will set the metric to a value of 1, and set the backup ADSL interface metric to 0.
The lowest value takes precedence, and the routing table is updated accordingly setting the default WAN interface to the ADSL interface. When the 4G interface comes back online the metric value is set back to 0, and the ADSL interface metric is set back to 1.
It works really well, and with OpenWrt, you can have many packages running that provide all sorts of extra functionality like Dynamic DNS updates, Private DNS, QOS and Bandwidth monitoring!
