1. check if the “Stubby” (DoT) is running or not and what port is listening to ?
    ( ps | grep stubby ), (if something wrong happens, please check system log to see what it complains about)
  2. If Stubby is running ok, please check the current DNS setting (Luci → Network → DHCP & DNS → DNS forwardings (delete the unnecessary ones and check the order of listing (if multiple DNS forwarding applied)