As it's currently not clearly stated in the documentation or in Construct 3, here is a handy tip for testing and preview your multiplayer projects locally.
Yes, you read that right, locally, without remote preview anymore.
I found this - not very well known and not so new - trick a little while ago on the C3 suggestions & ideas platform after very long and painful sessions. :')
Source: construct3.ideas.aha.io/ideas/C3-I-314
Good development and thanks for this feature Scirra!