Regulations of visiting Crimea is defined by the Cabinet of Ministers of Ukraine Resolution № 367 from 4 June 2015. Full text in Ukrainian is available here https://zakon.rada.gov.ua/laws/show/367-2015-%D0%BF
More details is available here https://dmsu.gov.ua/en-home/services/documenting-foreigners/granting-of-special-permits-to-enter-crimea.html
Any other ways to enter Crimea are illegal from Ukrainian laws and regulation and not recommended.
There are no any mobile operator roaming on the Crimea. But you can buy local SIM-card to have internet access.
Also there are no credit card payments support and only cash is in use (russian ruble). It make sense to exchange currency before enter the Crimea by better rate.
The best seasons for Super Randonee in Crimea are spring or autumn