When I got home, websites suddenly started showing certificate warnings on my iPhone, including Google and my college's Brightspace portal. A video had also briefly stopped loading just before this began. The same warning appeared in multiple browsers. Disconnecting from the home Wi‑Fi makes everything work normally, and my mom's phone has the exact same problem, so it seems to affect the network rather than one device. Nobody knowingly changed the router settings. What could cause this, and how can I fix it?
4 Answers
A failing fiber or cable connection can also cause the router or ISP equipment to behave strangely, especially if the issue started suddenly. If a full restart doesn’t resolve it, the provider should check the line and the modem’s configuration. The fact that your mom sees the same issue strongly points away from a virus.
Since multiple phones are affected only while connected to the home Wi‑Fi, this is probably a router or ISP/DNS problem rather than malware on your phone. Restart the modem and router, then check whether the router firmware is current. If the warnings continue, contact your internet provider and report that HTTPS certificates are failing across the connection.
Check the date and time on both phones and make sure they’re set automatically, since an incorrect clock can make valid certificates appear expired or invalid. If the time is correct, forget and reconnect to the Wi‑Fi, and test with cellular data to confirm the difference.
The router may be handing out a bad DNS server or redirecting requests. After rebooting it, temporarily try a trusted DNS service such as 1.1.1.1 or 8.8.8.8. Don’t bypass the certificate warning or enter passwords while it’s happening, especially if the warning identifies a strange or unrelated domain.

Related Questions
Can't Load PhpMyadmin On After Server Update
Redirect www to non-www in Apache Conf
How To Check If Your SSL Cert Is SHA 1
Windows TrackPad Gestures