Ethernet capability of 2014 Mac Mini

[Henry Tuesday]

My late 2014 mini has lost ethernet capability. It says "connected", but does not connect to websites. I bought 2 USB-RJ45 dongles which work fine on my phones, but still will not connect on the mini. I suspect there is a crack or something in the motherboard as it will go online for a few minutes sometimes when it is cold, but soon drops the connection.

My question: Will a Thunderbolt 2 dongle work or will it also say connected, but fail to connect on Ethernet (RJ45)?

Oh, before you ask; yes I have installed a new ethernet CAT 8 cable. Does not work any better than the original CAT5e.

AND I also have changed the Modem. Is it possible the fiber link box is causing this?

[Henry Tuesday]

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