Is investing in cryptocurrencies permissible or forbidden, especially given the variable administrative fees on buying and selling on foreign websites, and are these fees considered interest or wages, and is this type of transaction considered speculation?

There is no harm in trading currencies online if the prohibitions are avoided and the Shariah controls regarding currency exchange are observed. There is no objection to the fees that the company charges for buying or selling if they are known at the time of the contract, whether it is a fixed amount or a percentage. These fees are considered a wage for the company's services. This method is an agency for others in buying and selling for a wage, and not a Mudarabah (profit-sharing partnership).