Why do local network devices stop working when a VPN is turned on?

THE CLIENT INTERCEPTS TRAFFIC INTENDED FOR LOCAL IP ADDRESSES.

By default, a secure connection routes all network packets into the tunnel, isolating your device from the local environment. As a result, your computer or smartphone loses access to home printers, network storage (NAS), and other gadgets.

Local Range and Default Gateway Conflicts

Once a virtual network adapter is created, the VPN client alters the OS routing table, setting itself as the primary gateway for all address ranges. If this interception extends to private subnets (192.168.x.x or 10.x.x.x), local broadcast packets are sent to the remote server and dropped there.

How to Restore Access to Local Devices

Look for the "Local Network Sharing" (or LAN Traffic) option in your VPN app settings and enable it.
Add your home subnet range to the split-tunneling exception list.
Check if the hard network isolation feature (Invisibility on LAN) is turned off in your client security settings.