"...wondering if I could cast my phone to my roku tv without wifi and hotspot"
You do not technically need internet**, but you will at least need wifi so that you can connect both your RokuTV and your phone to the same network in order to cast from phone to RokuTV (local content on phone). If you had an ethernet adapter for your phone, then you could run both off ethernet and not need wifi. This would require at least a router. Or at least a spare laptop that you can configure to serve as a wireless router.
Wifi is basically just a wireless signal that allows devices to communicate. Some assume you need internet for wifi, so just clarifying for you that if you have a router, (even without internet service), most likely you have wifi and can cast. However, without internet, you will only be able to cast local content/storage from your phone to the RokuTV.
**Some Roku devices are unable to connect to a network without internet access due to Roku OS updates that is coupling the network access test check with the internet test check. If the internet test check fails, then may not connect to the network.
Roku Community Streaming Expert Just another Roku user... I am not a Roku employee. Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102
In addition to what @AvsGunnar mentioned, there are some other considerations.
Roku supports the following protocols: DIAL (casting), Miracast (mirroring), ECP/Play on Roku (casting), and AirPlay (casting/mirroring).
Its not clear whether you are trying to cast (from an app) or mirror (from the OS) from your Samsung A12.
To cast, simply select the "Cast" icon within the app (e.g. NetFlix/YouTube/YouTube TV all support DIAL) - the app must support the DIAL/AirPlay/Play on Roku protocol to cast to a Roku.
Mirroring may be more difficult, and since you mentioned your A12 lacking SmartView, we'll presume you want to mirror (using the Miracast protocol, which both Samsung and Roku support).
You'll need to download a third party app to either enable (disabled) OS-level Miracast-based mirroring, or add/use its own Miracast-based mirroring capability.
I recommend the Quick Settings app for Samsung missing SmartView (it allows you to enable hidden/disabled Quick Panel settings, such as "Cast", which will allow you to mirror using ChromeCast and Miracast protocol):