That is literally fucked.
There are some scenario's where I can imagine an ISP wanting to force wireless on. a mesh network for their customers sounds like the most straightforward reason.
if you cannot replace the router, faraday the shit out of it. put your own router behind the isp router and don't forget to change the MAC address of the router (isp will probably block any 3rd party router macs on the network. seen it before)
Like the peer comment mentioned. just drop nginx in front and let it so the TLS handoff.
always recommend to put nginx in front of any open source docker project as you can finetune many of the security controls there.
I suspect this is a remnant of the British era. Don't quote me on that.
In Belgium we have yellow panted stripes to indicate you're not allowed to park. Similar.
I'm using DAVx which is available on F-droid. syncs contacts and calendar with nextcloud.