Yes you can use a phone but then your phone will drain rapidly unless you USB tether it in which case your phone is literally tied up.
Also with a phone there are no wired ports.
On my sim router I find use for two ports.
I see a phone as a temporary solution.
A sim card router can become your primary net access solution to use instead of DSL or fiber.
There are SIMs with unlimited data.
I was imagining this more as a service mostly for expats.
Part time expats in particular don't need to commit to a more expensive wired solution.