Sounds far more likely that either someone misunderstood that residential IPs change frequently/may be shared by multiple subscribers or the ISP made an error when responding to a subpeana and provided the incorrect IP. Unfortunately both are all too common with privacy enforcement

If you really think the ISP router is snooping and can’t by bypassed you could simply double-NAT your network with a trusted router and call it a day. Much less VPNing and much less unusual decisions of trust and threat model involved then