Are you sure your “provider router” has given you a Public IP address? If your ISP is giving you an IP in the private IP address space you will never be able to reach your “provider router” IP address when starting the communication from the Internet.
About your second question, The WAN IP of your MT300 is 192.168.1.120. If you want to be able to open the Admin Panel via web when typing that IP on your browser, you need to go to Admin Panel > Firewall > Open Ports on Router and enable port TCP 80.
Does your main router have a public IP address? You should look at the IP address from your main router, and compare with https://www.whatismyip.com/, to see if they are identical.
Thanks, opening port 80 on the GL-inet now I can see the admin panel of the VPN router from the 1st ISP modem router. But how safe is it opening port 80?