Your R6800 is a MT7621 device. This ramips target has been converted to DSA.
Instructions for setting up VLANs with DSA are found here:
https://openwrt.org/docs/guide-user/network/dsa/start
There is also quite a bit of good information and discussion in this thread:
mini-tutorial-for-dsa-network-config
This information covers what you would like to do and is also good general knowledge for expanding or changing your network in the future.