They can use a shared computer, such as one at a library, or opt-in for alternate methods of participation, such as a postal vote. A physical polling booth may also be available to them, if officials still set them up alongside digital methods of participation.
We don’t tell our customers “you must use digital technology exclusively”. We are a technology vendor, providing additional options to them. It’s incredibly unlikely that overnight governments will decommission all centralised, physical methods of participation, and only allow people to vote using a computer or smartphone.