Hello @gary47
May I know has the plug been working fine before ? If it worked fine before, please try a factory reset on smart plug and install it from beginning.
How to reset TP-Link Kasa smart switch and plug?
If it's a new plug: Please try some suggestions in guide here:
1. Why can’t I enable Remote Control on the Kasa APP?
2. Change router DNS to 8.8.8.8, save and reboot router
3. Check if router has any advanced settings like Mac filtering, access control, parental control etc.
4. Try a different Mobile Service/ non-local Wi-Fi network like café' Wi-Fi, office Wi-Fi . to isolate if it is with certain SIM ISP/ certain network
5. Confirm Kasa app and firmware of kasa device are both up to date
Can I know the model of router, the network topology; e.,g. ISP XX- router model <Wi-Fi> kasa plug; did you try a different WIFI/ mobile data? Thank you