Add on 3G capability to router

[SAguy]

I have a adsl router at home that doesn't have capability to add on a 3G usb modem for failover. (Archer D7)
At the moment my ADSL line is dead (thanks Telkom) and I'd like to somehow add on the capability to change over to mobile data.

Not sure if there is a device that accepts a SIM that I can plug in to a LAN port on my existing router to share the 3G connection to all devices on the network.

Or would it just be cheaper to buy a new router that will allow for a SIM or usb 3G dongle to be added?
Looking to minimize the amount of setup needed as a few of my devices have static IPs and all that.

Failover doesn't have to be automatic - would actually prefer if I had the option to control it.

Any advice?

 

[Randy_The_Squirrel]

This is how the Squirrel does it because the Squirrel is sick of Chinese routers that are flaky.

Items Required:
1. Mikrotik Routerboard with USB port
2. Decent USB modem (avoid a HUAWEI modem that cannot be forced to default to a modem) you need to disable that built-in CD-ROM drive that has the drivers. I cannot remember what the name is for that feature, sorry.

Configure 1 to use the USB modem and connect with it.
When ADSL is dead, unplug ADSL router and plug in the Mikrotik

 

[RoganDawes]

I have one of those chinese routers from aliexpress, which has a built in LTE modem.

I installed the latest OpenWRT on it, with luci-proto-3g, and added the 3g device, with a metric of 20 (this controls whether packets are routed over the 3G interface by default or not). If my primary connection is down, ssh to the router, down the interface, and the 3g interface takes over.

Bring up the primary interface again, and the 3g interfaces recedes into the background.

 

[Randy_The_Squirrel]

None of my hardware can run OpenWRT so sadly I used the solution I described above