So let's try to think around the problem.
As I understand it, from your original description and your subsequent diagram, your Android phone is both your Internet gateway and a host for some applications that you want to access from other devices on the network:
Note, the LAN IP addresses are representative for the purpose of discussion; don't worry if yours are different.
If I understand correctly, one application hosted on your Android phone is Plex, and the Web UI listens on (I assume) port 8888. So your challenge is to be able to connect to http://x.x.x.x:8888/ but x.x.x.x changes every time.
You could, with some experimentation, configure OpenWRT to intercept http://192.168.1.1:8888/ and redirect the traffic to the Android phone.
The tethering gateway address isn't fixed every time, so port forwarding from OpenWRT's LAN interface to Android will be tricky to achieve directly. But what else could you do?
This is where obtaining the Android phone's IP address programmatically within OpenWRT might help, with a little bit of ingenuity.
One possibility which comes to mind is using iptables or nftables (depending on your OpenWRT version) to intercept and redirect traffic to the intended destination. If that approach is possible, it would involve some work with scripts on the command line.
One example, using iptables:
Detect the WAN gateway (next hop) once only, on OpenWRT boot:
/etc/rc.local:
# Put your custom commands here that should be executed once
# the system init finished. By default this file does nothing.
GATEWAY=`route | awk '/default/ {print $2;}'`
iptables -t nat -A PREROUTING -p tcp -d 192.168.1.1 --dport 8888 -i eth0 -j DNAT --to-destination $GATEWAY:8888
exit 0
You may also find success using crontab to schedule a regular iptables command, but bear in mind that the above example will keep adding PREROUTING directives; you may need to spend some time working out how to remove unneeded PREROUTING directives so that you end up with only the one(s) you need. One possibility might be to restart the firewall service each time, which will flush the table, but that's a bit of a crude blunt instrument and might interfere with other traffic which is flowing at the same time.
You might have success with some form of link detection, so that iptables runs only when the WAN link status changes. Again, as above, you'll need to find a way to detect and remove unneeded iptables entries.
The latest versions of OpenWRT no longer use iptables, but use nftables. The syntax for nftables is different to iptables, and I have not yet spent enough time with nftables to be confident of offering specific advice on it. However, the principle (intercept 192.168.1.1:8888, redirect to Android phone) is the same, even if the exact syntax to achieve it is different.
It may be an exercise for the reader to learn nftables and, in turn, provide advice on nftables to the rest of the forum...