(quick google about BGP sounds like it's some sort of heavy protocol - maybe there needs to be some way developed to get a single IP across different ISPs using 2 decicated usb wifi+LTE hybrid sticks)
That is still one box and one power supply. I'm talking about 100% redundant system and preferably mobile one. Probably the most solid solution is 2 laptops (due to each having own battery) and software BGP with 2 ISP's over multiple wireless connections - while you might want a well cooled usb wifi with external antenna connector - it doesn't really matter if the redundancy is solidly implemented such that transient failures of the usb wifi sticks only get registered in 'action center as to-do maintenance items' but do not affect performance.
This solves the problems you get with routers which are:
- software that might stop updating once router vendor deems product 'legacy'
- reliance on single power supply (secondary caps in tight space near the parts that heat up - built in time bomb)
- no battery backed power
- who wants to lug business routers around if they want redundant connections while mobile?
- nothing in the router box that cannot be run on idle core without loading system up too much, it's just less powerful computer that is less guaranteed to receive upgrades.
- with software BGP support you should be able to get the single IP across two wireless usb sticks connecting to different operators - this might be doable without BGP but I haven't yet heard how ...
- BGP may have problems in customer (possible attacker) facing scenario but I can't really tell, it just sounds like something the author of this blog is concerned about: http://blog.ip.fi/