Check if your Space Engineers server is online
Space Engineers answers a Steam-based query on a port of its own, apart from the one players join through.
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The ports Space Engineers uses
Both are UDP. Out of the box the query port and the game port have different numbers, and it is the query one that this page and the server browser use — which is why forwarding just one of them leaves a live server that nobody can see.
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Still offline? Try this
Still failing with both ports open? Loading may not be done. At boot, Space Engineers recalculates physics for every grid in the save, and a world with many large ships can be busy with that for quite a while before replying.