Somebody sent me this message: Hey there,
I think I found a solution for your problem. There is a reddit post describing pretty much the same problem but it also has some solutions that were presumably proposed by the stellarport customer support. You can find it here:
I don't know how versed you are with computers/programming but for method 1 you would have to do some things manually. First, you would have to find a program that can derive accounts from your seed phrase. This would assume that you still know the seed phrase, i.e. the 12 words that were shown to you when creating your account on stellarport. Then you can install something like this software:
https://github.com/reverbel/seed-phrases-for-stellar#2-same-example-as-above-now-showing-the-bip-0039-seed-and-four-accounts, enter your seed phrase and look at the first and second account. According to the reddit post, the second account is a co-signer to your primary account, thus you can import both into a wallet app that supports multi-signature features (e.g. Solar), then change the multi-sig setup of your primary account and remove all co-signers, and then this transaction should appear on the second account where you can finally sign and submit it to the network.
If this seems too complicated then you can try method 2 and write the stellarport support team.
I hope this helped.