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.
I'm having trouble getting my devices to connect to each other over WiFi.
For example both my computer and printer are connected to my router but when I send something to print, it doesn't seem to connect, troubleshooting finds no issues and both devices show strong connections to the WiFi.
I run into similar problems when I try to use my phone to connect to my Roku, with both the phone and the Roku in close proximity to the route, but they will not talk to each other a majority of the time.
I seem to remember reading something about a possible issue with security setting on the modem when I was trying to get my Roku app to work, but I cant seem to find it now.