I recall successfully renewing my registration several times online before, but now they want a phone number and they are pushing some weird government smartphone app "to handle your government todo items"?
The paper mailer directs people to the new and shiny https://www.texas.gov/ (which wants you to create an account and have a smartphone, etc).
The old webflow I mentioned is here https://www.txdmv.gov/osp-registration-renewal (which does not require a mobile number or account creation, etc).