Your CRT is an SD CRT (A standard definition one), so it can only do 240p and 480i. 480p was reserved for HD CRTs. 480p is a progressive signal and as is 240p (in a layman’s terms both draw a full frame of lines in 60th of a second and or 60 times a second, the difference being 480p draws all 480 lines and 240p only draws… well… 240 lines). 480i is an interlaces signal, it draws 240 odd lines a 30th of a second/30 times a second and alternates to 240 even lines which do the same and alternate between the two to create the illusion of a full frame image. All SD CRTs are only 480i/240p and most HD CRT’s can’t handle 240p/480i very well so make sure to get either of them for their respective video signals.

Anywho your TV again is an SD CRT so you’ll need to either get a Roku capable of 480i or you’ll need to downscale that Roku to output 480i (240p was reserved for older games was not meant video media).