If you are having trouble connecting LiveU Solo to Castr via SRT protocol, you must ensure:
LRT subscription is required for SRT to work: https://solohelp.liveu.tv/hc/en-us/articles/13933926074395-How-Can-I-Setup-An-SRT-Destination
If it still does not work, then choose a LiveU location which works with SRT: https://solohelp.liveu.tv/hc/en-us/articles/13933642078235