Will be able to change the LAN IP address soon with new firmware release. Does your modem change anything that may cause the Deco to change the DHCP?

check whether there is public WAN IP address and gateway on the router when connection drops. change DNS to 8.8.8.8 if yes. Verify your modem is fine. otherwise, contact tplink technical support. If...

since it's UN version, you can downgrade to older UN version

strange! I think it's better contact tplink technical support directly.

reboot the router, try update again. it's not easy to get it done. try a few times.

Use the same way to downgrade the firmware for V1.