Thanks but it doesn't seem to have worked in my case - unless sites are throwing me off for a different reason.
How did you test it?
Just to clarify the exact steps I took:
1. Logged in via SSH
2. Edited the mwan config file:
# Added the following above the "default_rule" block:
config rule 'https'
option sticky '1'
option dest_port '443'
option proto 'tcp'
option use_policy 'balance_default'
3. In the web GUI, turned Load Balancing off for my two modems, saved, then back on again, saved
Edit: changing rule name from "make_https_sticky" to just "https"; got this working.