If I understood the problem properly, you are not using Ledger's Theta wallet but a different one. From what I can see on Ledger's official website where they are displaying the
supported crypto assets, Theta is natively supported on Ledger Live. If you can't connect whatever wallet you are using now, you could try to move your coins to Ledger's Theta wallet. You already got the hardware wallet, might as well take full advantage of it.