I'd suggest using a spare old cellphone permanently plugged into power as an access point.  It will have to be connected to the internet through a data connection, (requires a SIM and a Data pack deal), & running a permanent 'hotspot' from the phone.  That would give you a connection for the cameras independent of the 'house' connection.  Whether this is practical I'm not sure as the cams would need to be permanently connected to it. 

I have a couple of Xiaomi cameras and while they 'can' indeed backup all data to the cloud I don't do that I simply have an SD card in each.  The cam would work independant of whether there is an internet connection or not, (well i'm pretty sure of that), but you need a connection of some sort to remotely access the cam...........and to get alerts etc.