For this you would need a script that checks if the fibre connection is established or not, if it's established - it disables L2TP, if it's not - enables L2TP back like it was. Ways to check it would be these:
1. Pinging through the Wired WAN interface and checking if ping succeeds.
2. Downloading a third party opkg package that checks port status.
3. Always actively checking logread output if Wired WAN connection is established.
How to write/use scripts - you can find here:
Unfortunately, we will not be able to help you write them as this would need to go to our RnD department and development of this by us would not be free.
Hope this was somehow helpful to you.