There is apparently a way to set up a bridge that will allow you to access it, but that sounds like an awful lot of work. It also requires connecting to a PC running the software, and I would imagine it affects the security of the messaging (which may be the reason to choose proton mail in the first place).
https://proton.me/support/protonmail-bridge-clients-windows-thunderbird
Can someone ELI5? What is the problem that this solves?