CalcKey is awesome but currently almost no mobile apps support it. It runs fine in a mobile browser but sometimes you miss having a dedicated client for it.
Give https://kaiteki.app/ a try. It's still in Alpha but honestly day-to-day its stable and does the job with little fuss.
It's built for MissKey, CalcKey's poppa, but handles CalcKey very well too, as far as I can tell. It seems to be a simple web app, without...any...settings once you get logged in, but that invisibility just lets CalcKey do all the shining. ☺️
Oh, and Kaiteki means "comfortable/comfy" in Japanese and so far that seems very apt. (Apped, geddit?) #CalcKey#MissKey#Kaiteki