One of the rlm_yubikey module’s functionalities is to check yubikey OTP token values against a cloud-based Yubico validation server.
Yubico OTP is a strong authentication mechanism, that can be used without any additional client-side software. You need Yubikey series 4 or 5. They just work as a USB keyboard to the operating system. No need to install any special drivers. The principle of operation is described here.
An example otp is as follows: ccccccukiegehhulguubvvcufvlnelicklfitvndnkeu.