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An Internet Service Provider (ISP) can block access to a mobile app using several methods, even if you have already downloaded the app. Here are some common methods ISPs might use:

- IP Blocking: ISPs can prevent access to the servers that the app communicates with by blocking the IP addresses associated with those servers.
- DNS Filtering: By altering the Domain Name System (DNS) responses, ISPs can stop the app from resolving the server's domain name, effectively blocking access¹.
- Firewalls: ISPs can use firewall rules to block traffic to and from the app based on various criteria, such as the destination IP, port numbers, or other protocol-specific details.

To bypass these restrictions, users often resort to methods like using a Virtual Private Network (VPN), which encrypts your connection and routes it through a server in a different location, or changing to a public DNS server. However, it's important to note that circumventing ISP restrictions may violate terms of service and local laws, so it's crucial to be aware of the legal implications.