local bitcoin nodes in a same vlan cannot sync
Then I delete the static dns seeds in file chainparam.cpp for both two nodes. So they both cannot connect to the bitcoin network.


They can still connect to the Bitcoin network without the DNS seeds. The DNS seeds are not the only way to connect to the network; there are numerous fallbacks, with DNS seeds themselves being a fallback too.

-datablock= is not a valid command line option. Did you mean to use -datadir?

Check in the debug window of both of your nodes; does node1 have a number of blocks that you expect it to have? Does node2 already have blocks that it downloaded very quickly and you didn't realize it?

Instead of using the addnode RPC, try using the -addnode or -connect startup options to connect your nodes.