Can't add IP filter to lightsail network console

I suspect you are going hitting a limit based on the IPs in your rules. Take a look at the following page.

https://repost.aws/knowledge-center/lightsail-considerations-for-use
13. Number of Lightsail firewall rules: You can add up to 60 firewall rules for a Lightsail instance. Each source IP address is considered a different rule. IPv6 rules are counted towards this quota of 60. The maximum number of source IP addresses that can be added in a rule using the Lightsail console is 30. To add more, you can use the AWS CLI.

Look into this document for more details about specifying IP address range for the firewall rules: https://lightsail.aws.amazon.com/ls/docs/en_us/articles/understanding-firewall-and-port-mappings-in-amazon-lightsail#specifying-source-ip-addresses.

If the IP address range you are specifying is part of a previous rule, it won't show up as it's already a part of one of the rules. And if the IP addresses are next to each other, the entries might merge into a single one. For example, if you add a rule with 192.0.0.0/24 and another one with 192.0.1.0/24, they show up in a single entry as 192.0.0.0 -> 192.0.1.255. Make sure that the new IP address range that you are adding is not a part of a rule that already exists and verify if it is getting merged into an already existing rule after adding.