to ensure that I am reading this properly, can you confirm that the Ubuntu Server has Wireguard setup in a server-type configuration (in other words, it is listening for inbound connections from the phone and the OpenWrt peers)?
If so, this shouldn't be that difficult to achieve.
Let's take a look at the config files from your OpenWrt router as well as the WG config from your phone.
Please copy the output of the following commands and post it here using the "Preformatted text </> " button:
Remember to redact passwords, MAC addresses and any public IP addresses you may have: