Unfortunately, you cannot deduct the cost of internet service for teaching from home.
Part of the Tax Cut and Jobs Act of 2017 was to eliminate almost all deductions for unreimbursed employee business expenses.
Unless you are in a very small group of employees, the amounts you enter will be reduced to zero on your federal return (although some states still allow the deductions, so it's good to enter them).
The exceptions are: Armed Forces reservist, qualified performing artist, fee-basis state or local government official, or an employee with impairment-related work expenses.
TurboTax will ask if any of these apply when you fill out this section. You are encouraged to enter your expenses no matter what your occupation as the expenses my be deductible on your state return despite not being deductible on your federal return.