The most common causes I run into are:
1. Client Isolation. Can be called by other names in routers.
2. Firewalls. Some firewalls block devices on the LAN. You have to test with the firewall off in the router. Sometimes you get new networkers that are scared about this setting. This is a rather new feature in routers (less than 10 years old) and what it does can interfere with LAN operations. Your Internet firewall is the same as prior due to how NAT works.

3. https://www.ic3.gov/media/2018/180525.aspx notes an issue with VPNFILTER or the only known widespread router hijack. There are more articles about that and should be rare to find a router with this issue but I'll add it anyway.