504 when mirroring upstream repo (after clone/push)

I want to use GitLab to mirror my main repo, so I will only be pulling into GitLab from my main repo.

I read the notes that said “The update action will time out after 10 minutes. For big repositories, use a clone/push combination.”, so on my local machine (which is not the machine which holds my main repo) I cloned master and pushed it to a new project on GitLab. That seemed fine.

I then set up GitLab to mirror my main repo using https. When I click ‘Update Now’ I get:
error: RPC failed; HTTP 504 curl 22 The requested URL returned error: 504 Gateway Time-out fatal: The remote end hung up unexpectedly

I think I have done everything that the docs say I should have so am puzzled why it is not mirroring.

Can anyone see what I have done wrong?