hAP AC, RouterOS v6.43.11.

I have Ethernet access speed from my ISP at 1Gbit/s. It worked for a long time but one day I noticed it is 100Mbit. I changed my RJ45 plug to the new one and the issue has gone away. Some time ago I noticed my hAP AP connects at 100Mbit/s again. I'm trying to reconnect the cable, see the 1Gbit connection for a few seconds, then 100Mbit again. I've just tested the connection with an operator on a different device (Mac mini with 1Gbit port and it worked as expected.)
1. Where might be a problem? I didn't change any settings from that time when the router worked at 1Gbit.
2. Is it possible to assign the WAN port to another than #1 to check if there's a problem?

Thanks.

Cable length?

Cable length?

To nearest switch I suppose? When I unplug and then plug Ethernet to WAN port, I see in Cable Test window ~103-105m. Why? What has happened from that time it worked properly? I don't think they moved a switch farther from my home.