Generally, the router doesn't impact connectivity between devices on a single subnet... there are a few things that could cause these problems like wifi client isolation.
Let's see your config files:
Please copy the output of the following commands and post it here using the "Preformatted text </> " button:
Remember to redact passwords, MAC addresses and any public IP addresses you may have:
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall