By international agreement, there are only three IP address ranges that should be used within private networks: 10.0.0.0-10.255.255.255, 172.16.0.0-172.31.255.255, and 192.168.0.0-192.168.255.255. If one choses to use a non-private address range, one must accept any possible limitations or problems that might occur due to their use. Your use of a public address range means to have to live with the issues that arise from that, and the unavailability of the Roku app is one of the limitations.
There are many good reasons NOT to use public IP addresses on a private network, but I'm not going to discuss them here. However, I will mention one reason that is probably the most important: IP packets originating from or addressed to a private IP address cannot be routed through the public Internet. When you use public IP addresses, it's possible for your network to be accessible from the public Internet, and that's not a good thing security-wise.
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