1. Turning firewalls and antivirus off makes no difference, so it ain't that.
2. If it was DHCP allocation then disconnecting various bits would cure it, but once the routers have gone they won't even work with only one computer on.
3. Therefore it also can't be an ISP limitation (and anyway the ISP will only see the router, nott he items my side of it).
4. New expensive routers last no longer than cheap ebay ones. The fact that so many have gone makes it unlikely that the mtbf of cheap machines may be shorter than the more expensive ones.


All worth thinking about, but none likely to be the answer yet.