They're only killing the crappy store/UWP version that nobody used anyway and only caused confusion. The normal OneNote bundled in Office isn't going anywhere as far as I know.
That said, I've moved a lot of my note taking to Obsidian. It's not a perfect replacement but it's a fantastic markdown editor and now I use both for different use cases.
The easiest way that doesn't affect the main network would be to use a travel router. Its WAN IP would be the private IP it gets from the main network (over wireless since that's your only option). And it would NAT your network onto that IP and then you can do whatever you want on your network.
I'm not sure if that Mikrotik router will do this but it might. You basically need something that can connect to an SSID and use that interface as its WAN interface. The wireless factor here is really limiting your choices. If you had a wired uplink to the main network you could use any router/gateway/firewall you wanted. You could also use an AP in bridge mode to connect to the main network's SSID and wire it to the WAN port of any router of your choice.
You don't really need to use VLANs to separate your network from the main network unless you want to share any of the same layer 2 segments (basically wired Ethernet) while keeping it isolated. But it doesn't really sound like that applies in your scenario. Of course using VLANs within your network would still make sense if that applies (for example, to separate your server traffic from your IoT traffic).