there's nothing in the default openwrt config that would stop a device from making DoH requests (it'd would block all HTTPS traffic in the process).
if it doesn't work, you've probably made a config error
Please connect to your OpenWrt device using ssh and 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:
ubus call system board
cat /etc/config/network
cat /etc/config/wireless
cat /etc/config/dhcp
cat /etc/config/firewall