Zoom: Currently free, easy to use with a computer or smartphone, lots of great features (e.g. whiteboard, screen sharing, chat). There is a learning curve, though, and students would likely need training on how to use it. https://blog.zoom.us/wordpress/2020/03/13/how-to-use-zoom-for-online-learning/
Google Classroom - https://classroom.google.com/h : Free; highly recommended by many educators; accessible by computer, phone, and even game consoles. Many schools already have Google accounts for students, which makes this easy to access. See the guide for teachers ( https://teachercenter.withgoogle.com/first-day-trainings/welcome-to-classroom) and guide for administering assessments ( https://support.google.com/edu/classroom/answer/9095575?hl=en ).
Teacher-directed phone calls: Allows teachers to connect with students 1:1, no internet required. Teachers could create a Google Voice phone number ( https://support.google.com/voice/answer/115061?co=GENIE.Platform%3DDesktop&hl=en) for students to call and text. Calendly (https://calendly.com/ ) is a useful tool to quickly schedule 1:1 check-ins.