Sometimes, it jumps to 100 Mb/s, I don't know why Luci can't configure the ports to the required link speed as it happens in many switches.
I tried with "ethtool -s wan autoneg off speed 1000 duplex full"
but it only lasts a few seconds and goes back to autonegotiation.
1000mb is only auto-negotiation, you cant force gig due to ethernet official specification
99.99% chance it's a cable issue, other 00.01% is a switch, or somehing else like power-saving trash
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If speed is forced to 1000 but the link cannot operate at 1000, it will disconnect completely. This is probably not what you want. Identify the cause of the speed fallback. It can be a bad cable or bad hardware at either end. Modems in particular are prone to be damaged by lightning EMP coupled in on the cable or DSL line. This can cause ports to not have full performance.
I changed cable and it has been a long time since I was down to 100 Mb/s. Even if you can't force 1000 Mb/s, why isn't there a menu to force 100 or 10 Mb/s?
Well that's one way to know that the link speed has gone down 
If I reboot or restart the WAN link in Luci I go back to 1000 Mb/s but since I changed cable for a long time I did not go down to 100 Mb/s, I have to say that the router is not next to the cable modem.
Check EEE as issue
What is your device ?
I have an old 3700v2 on which the WAN (1GB/s) suddenly dropped down to 100Mb/s without explaination. I have checked the pins, cable ... no way.
I don't understand that, what do you mean?
Energy Efficient Ethernet, can this be configured in Openwrt? not in the Cablemodem.
root@OpenWrt:~# ethtool --show-eee wan
EEE Settings for wan:
EEE status: disabled
Tx LPI: 30 (us)
Supported EEE link modes: 100baseT/Full
1000baseT/Full
Advertised EEE link modes: Not reported
Link partner advertised EEE link modes: 100baseT/Full
1000baseT/Full
Newifi-D2 MediaTek MT7621 ver:1 eco:3 OpenWrt 22.03.2