This is not the wisest purchase I have ever made! I should have done my research, but I purchased this telly because my cat broke my telly and the price was right! other than that I would have NEVER gotten this thing. You can't stream some of the apps that you should (tried to log into my fiton app). It continuously said that my email and/or password was incorrect. When I attempted to speak to a human being that was impossible and then when I did, they were useless! kept telling me the same thing the bot was telling me. I can't cast my android phone to the telly unless I pay monthly...WTF!!? I would never let anyone purchase this product EVER! In fact, if I could find my receipt I'd take it back without question!! Come on Roku, get it together!!
Relax, throw another shrimp on the barbie, and provide some details as to the specific issues you are having.
When you are trying to login to FitOn, are you using the same credentials that you would if you logged in to the website https://fitonapp.com/ or using the app itself. (do not use your Roku credentials, use your FitOn credentials). If having issues with this, likely will need to contact FitOn to see why their channel/app is not accepting the credentials. (https://help.fitonapp.com/en/?ref=header)
As for the casting, there are different protocols for casting content to the Roku. Basically, you can only really cast Youtube, Netflix, and a couple others directly to the Roku using the cast button on an Android. (no native Chromecast support unless integrated/provided by the channel itself. Technically, uses a DIAL protocol). Since the newer mDNS protocol has been figured out (reverse engineered), more apps should become available that will allow their use with Roku).
Depending on what you are trying to cast, you may want to try Web Video Caster and see if that fits your needs. https://channelstore.roku.com/search/web+video+caster. They offer both a free version and a paid version. (one time purchase of $3.99 I think).
Otherwise, you may want to purchase a Chromecast device/dongle and connect it to your RokuTV to expand its capabilities. Sometimes need to customize a setup in order to satisfy certain needs.
Feel free to post back if you haver any other question or need additional help.
To add to what @AvsGunnar said, one line in your message stuck out:
I can't cast my android phone to the telly unless I pay monthly...WTF!!?
This sounds like you called a scam line, not Roku. If you Googled Roku help, you may have selected a scam site. Roku doesn't charge for using their equipment.