If you are running JRiver for Android on a phone, JRiver BingoSSDP or the free PingTools can find the MC Servers and once that has happened on the phone, JRiver for Android should be able to see the MC Servers as well.
Both those Apps use SSDP to location the servers, and doing so seems to establish a network path in my router, which is stored for a while in the router ARP tables, so I can connect for a while after running those Apps, until my router flushes its ARP table, which usually takes from a few minutes up to 15 minutes or so, depending on the router.
Mind you, JRiver for Android uses SSDP to locate DLNA/MC Servers, while JRemote2 uses MCWS packets, so I usually find JRiver for Android can find MC Servers, but JRemote2 can't until I run one of those Apps, rather than the other way around. But today I had to run BingoSSDP to get JRiver for Android to see all MC Server. That is with JRiver for Android 26.0.111, which has other issues, so maybe something was broken.
Also, it can take a while for MC Servers to show up in JRiver for Android, and maybe a refresh (exit and re-enter) of the Load Library screen.