How to lock a router for a specific provider?

Question

Computer networks

Hello.
Take, for example, the Tenda n301 router.
Is it possible to lock the router for a specific provider?
How is it generally done? As far as I understand, the firmware is being modified, but what exactly?
Limiting the possible range of IP addresses that a router can receive?
Are there any firmware configurators? Which direction to look?
What language are the firmware written in? What if you take some open-wrt and modify it for yourself?