I don't want to spend megabucks on a 5G router and cellular contract just for a backup network, so I'm wondering if I could configure the following

Connect a MikroTik RBD52G-5HacD2HnD-TC (hAP ac²) to my Cloud Series router, and when the Fibre company is doing maintenance start tethering on my Android phone and use the hAP ac² in station mode to route to the internet via the phone.

I know that it won't be possible to do IPv4 port-forwarding, but using an IPv6-over-v4 tunnel I should still be able to get port-forwarding to my mailserver over IPv6

Can anyone let me know if this sounds sane? and is there any reason why it wouldn't work?

TIA Banjo

Works fine. You don't even have to use wifi, you can use USB tethering as well and keep the wifi interface available for other purposes.

My home router is a hAP AC2.

When my wired ISP goes down... I can turn on the hotspot on my phone. The hAP AC2 connects to my Pixel6Pro on the 5GHZ radio. Then my whole house gets routed through the phone's WAN connection.

However... My wireless carrier is intent on putting 5G to the home. When it's available in my area... I will pay the 240 bucks a year to have that as my always on back up.

When connect to my phone as the house WAN... I only get 20Gigs before I risk getting throttled.

Besides... I would subscribe to a binding service if I had 2 active ISPs. That way I could have combined bandwidth and immunity to connection severing when an ISP goes down.