The trustline for Solo drop is created on the XRP Ledger. XUMM app is only an XRPL wallet same as SOLO app. with either, you can interact with XRPL and create a Solo trustline.
IMO the "easiest" for someone just getting started with claiming Solo is going to be the Solo DEX wallet app
Install Solo DEX app, import or create XRP wallet, and it will also create a Solo wallet. Activate the Solo wallet (free) and now you are good to go for the drop- just be sure to have all your XRP in the Solo app's XRP wallet on Dec 24th for the snap.