this is a big problem, I ran into that difficulty because not even I can use that keychain app since it doesn't work on the phone, but they gave me a solution using stermlogin.co and they gave me the links to buy, sell and withdraw steem, with pleasure I leave you the links with their instructions.
You can use this link to buy. Change your username, price, amount, token symbol in the link.
https://steemlogin.com/sign/custom-json?required_posting_auths=%5B%5D&required_auths=%5B%22yourusername%22%5D&authority=active&id=ssc-mainnet1&json=%7B%22contractName%22%3A%22market%22,%22contractAction%22%3A%22buy%22,%22contractPayload%22%3A%7B%22symbol%22%3A%22EPC%22,%22quantity%22%3A%220.001%22,%22price%22%3A%224.00000%22%7D%7D
I hope this is useful to you, it is a cumbersome signature but it is the only that I have on hand and allowed me to move my coins in the steem engine.