I have a hallmark subscription and when I got to the channel on my Roku, the login screen never comes up. When I log in on my phone, everything works fine
model 3930rw
software version 10.0 build 4209-AE
device id S013209TOPFA
Are you using the correct Hallmark Channel/app on the Roku device? There are a few different ones available. (Additionally, I believe the Hallmark Channel that has live tv is "cable/tv provider required" (Hallmark TV) unless you use one of the cable alternative Channels like Frndly or Youtube TV). I believe you should probably be using Hallmark Movies Now.
If you are sure you are using correct Hallmark Channel/app and worked previously, you will probably have to contact Hallmark directly regarding their Channel/app.
Since you are using Hallmark App on your phone, sounds like you subscribe directly to Hallmark. In that case verify you are not trying to sign-in to the "Hallmark Channel on Roku" as this is a different subscription.