@Caspy, Here are some things you can do
1) use a high port number
2) Use the webservers built in "firewall"... Your mobile device most likely uses the same ip address ranges every time...
The Limit IP access box provides basic firewall capability for the web server. If left blank, or if there are no specific inclusions, all IP addresses are allowed to connect if not specifically excluded. To add an included IP address or range of addresses, just enter the IP address, optionally preceded by a + (plus). Use the * character in place of a number as a wildcard. To add an excluded IP address or range of addresses, precede the address with a - (minus). Separate multiple addresses with either a comma or a semicolon. Here are some example:
+192.168.*.* allow all typical LAN addresses
-58.*.*.* deny any address beginning with 58.
+68.77.123.1 allow this one specific Internet address
Remember to always use a very strong password.
