Depends what your looking for , and where your wanting to take the training .
If you need it for STCW , QMT in JAX Fl. has a great instructor that teaches Radar , principle , theory , and operational aspects . Guy is good at teaching , if you do not understand it he will find a way to get it to make sense .
Otherwise there are a few books avail. for general knowledge .
As far as GMDSS goes , if you have no experience with radio equip or theory , start digging into any info that you may be able to find , cause the GMDSS class that is taught for STCW should be three times as long as it is for any one not acquainted with radio . The class is fast and includes a lot of stuff that may not make sense at first so it is easy to get left behind . It is also a bit expensive .
Neither subject offers any on line Actual STCW training that I am aware of . Only online study .
PM me if you need any additional info .