what will happen when there are some full nodes in the network which are not connected to one client-how can he get new blocks updates?
Information in Bitcoin is chiefly relayed by a peer-to-peer gossip protocol between the participating nodes.
Generally, transactions are submitted to the network by the sender's node or via any service that has an open API to allow transaction submission. There's on-going work on a proposal to obfuscate the original source of a transaction in a proposal by the name of "Dandelion".
On the other hand, new blocks originate from the authoring miner and are quickly disseminated via dedicated relay networks such as FIBRE, and in parallel relayed via the regular peer-to-peer network.
The latest blockchain data is also broadcast via satellite.
Lastly, nodes can be initially synced up to the network by receiving the blockchain data by means of a conventional hard drive. Only the very latest information would then need to be acquired from the network.
In short, if your node is not connected to any other peers or the satellite broadcast, it will not keep up with the network as it has no means to receive the latest information.