What she needs is a custom router with 3+ ports and dual WAN support. Basically by a cheap n100 box off Aliexpress for $150, slap pfsense on it, configure dual WAN and bam you're done -- seamless failover the instant one network starts dropping packets.
https://www.aliexpress.us/item/3256804173757529.html?spm=a2g0o.productlist.main.1.53e66897Mm6ku3