An increasing amount of users with the recent OS validation loop problem on the TI-84 Plus CE are visiting this topic but since it was originally about another similar issue I wanted to link to the TI Education page about what to do if you have a revision AA TI-84 Plus CE/CE Python calculator stuck in a boot loop when plugged in.
A few months ago, Texas Instruments released an older batch of TI-84 Plus CE and CE Python calculators, probably due to failure to rotate their inventory properly at one point in time and by the time those calculators (revision AA from 2022) made it to stores the batteries inside those calcs became completely discharged or dead.
Well, it's called Telnet but it's pretty much a serial monitor on the calc side like the one that comes with the Arduino IDE, no unix shell needed. It was designed for use with one but you don't really need one.
Yeah, from what I remember it's not easy for a calc to communicate with an Arduino due to timing issues and the CPU being barely powerful enough for fast serial communications, I'd advise trying with one Arduino first at low speeds with the build-in Serial library (iirc you can plug the TX and RX pins directly on the 2.5mm jack), and on the calc itself use a telnet terminal such as this one: https://www.ticalc.org/archives/files/fileinfo/141/14141.html