If it’s logs, there’s a package called log2ram - it’s designed for small form factor systems to reduce writes to SD cards but does apply anywhere you want to log but not hit disk immediately. It syncs logs to disk on a regular basis so you don’t lose much if the system crashes.
From a Linux command line it would be the command called arp, you need to add a static arp entry. I don’t know how that works on sense, but on Linux it would be something like
arp -s IP MAC
Maybe there’s a module in opnsense to help. The way I’ve done this before is using a machine connected to the same network at my target to wake up by logging into that machine and issuing the wake command.
WoL packets are usually sent to the ip broadcast address for the network as they’re not ip based. I don’t know if this would ever work well across networks. Can you do send the wol packet from the opnsense router instead? Does it work then?
If you’re sending it to the IP of the server, it likely works soon after your turn the machine off because the ARP entry hasn’t timed out yet, but once it times out it won’t work anymore. The router doesn’t know how to get to the machine. You may be able to add a static arp mapping to get it to work long term.
In general, if you haven’t taken steps to expose your service to the Internet, it’s not accessible over or to the internet. Your router that connects you to the Internet should have a firewall that blocks all inbound, unsolicited requests, and you also need to do something explicit with most self hosted service to expose them, they will not announce themselves to the world.
In addition if you’re using an ipv4 network address that’s likely a private address (like 10.x.y.z, 172.x.y.z, or 192.168.x.y), which also isn’t accessible outside of your network.
The phone or browser may be using DNS over HTTP (aka DoH), check if you can disable it for the wifi network. You may have to disable it on the phone or browser to get your desired behaviour - look up directions for your browser.