A RPi 4 has GbE and a proper installation would negotiate to use it.
That isn’t an issue with standard openHABian let alone openHAB but with your networking setup and/or hardware. You need to provide networking details and what you have analyzed and tried so far if you want others to help you; strictly speaking you should go looking for help elsewhere on some networking forum. Please at least change to use proper tagging on your post.
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