Would be expensive, unlikely to get carrier subsidy, hence unlikely to be a market.
3 box solution already available at modest cost, and more flexible: small mobile 4g/5g router with USB type power supply (creates a portable WiFi) and Roku device. Tape together with a 2 port power bank and carry a 3port USB power supply to power/recharge everything all at once.
Or have a second cheap mobile phone which offers tethering and use that to create WiFi hotspot for Roku.
Not a Roku employee