Your node when you first set it up will boostrap (ie.., discover peers) using DNS based on "seed nodes".

The ISPs may frustrate their users when installing a bitcoin node who are using the default DNS that is under control of that ISP, but workarounds are for the user to configure their node to use Tor, to use a VPN, or to configure their device's networking to use a different DNS server outside the jurisdiction of a specific government. And again, that is only for initial set up of a node.

Once a node is running it already has stored locally a collection of peers it will connect through. Those peers will be of various geographic locations (both IPv4 and IPv6), and various ports. The number of peer nodes your node could connect to will include over 10,000 nodes and the set of nodes sees new listening nodes come online daily.

So essentially, a ISP could be ordered to frustrate users of new nodes who are using the default DNS settings, and that ISP could do a limited DoS by blocking any traffic to addresses + port combinations that are discoverable by a node. All that would do is result in a user configuring their node to use Tor or to use a VPN for their node. And even for those who didn't use Tor or VPN, the ISP would not have 100% effectiveness as they are at best simply playing a constant game of whack-a-mole trying to block the new addresses + port combinations that would constantly be appearing.

Here's a question on the BItcoin Stack Exchanges with similar information:

Is it possible to restrict Bitcoin's network traffic? https://bitcoin.stackexchange.com/questions/106569/is-it-possible-to-restrict-bitcoins-network-traffic