The interfaces are auto-negotiated by default. The recommendation is to try as far as possible to use auto and enable auto negotiation on the switch/router side so that negotiation succeeds.
For the corner cases, when the recommended method is not possible, here is how to set it manually:
ethtool -s wan0 speed 100 duplex full
ethtool -s lan0 speed 100 duplex full
Once you are happy that the commands work as expected and you want the commands to run automatically after reboots etc, then add them to /etc/wanos/ethtool_options file e.g.:
ETHTOOL_OPTS_WAN0=speed 100 duplex full autoneg off
We'll usually do something like this for the client remotely, but you can run them, just make absolutely sure there aren't typos in the line.