I do not believe so though I suppose it depends on the ISP and model of your modem. Sure it's possible to have your modem spoof or fake it's hardware Mac ID infact most modems have a hidden ssh server or telnet servers you can hook into to re-coded it's built in web pages and administration pages. But I highly doubt you would get free internet by simply changing the modems MAC.
I know in most cases when a ISP blocks or disables your internet they usually do this via a DNS lock. They will force your modem settings to use a custom DNS that is locked to redirect any web request to there payment or notification pages.
In some cases you can bypass this DNS re-route block by editing and configuring a new DNS into the modem for example Google DNS. But this does not always work and if it does it's usually only for a few days.
If you have a modem that supports dialup and a active phone land line you can get free dialup just lookup free dialup ISP # but I mean my god sir do you really want to be using dialup in 2020 xD
Not that it's legal but you could always try using some WPS exploits or crack into a WEP/WPA WiFi that is within range for free internet.
But editing your router/modem configurations to get free internet is very unlikely
