This certainly sounds like an issue with that switch and not your Mi 4A with OpenWrt. The way to verify that the Mi 4A is working properly is to use another ethernet device to see if a link is established on each of the LAN ports (and it sounds like you may have already done that). If the result of the test shows that the 4A is okay, you will want to replace that switch.
If your problem is solved, please consider marking this topic as [Solved]. See How to mark a topic as [Solved] for a short how-to.