I previously accessed my Ledger BTC wallet through the Magnum wallet (
https://github.com/magnumwallet), but Magnum has recently shut down and now I can't find the address on my Ledger.
I will not recommend such wallet to be used as an extension of a hardware wallet. If truly magnum wallet is used as your ledger nano extension, that does not mean it is not the ledger nano that generate the seed phrase, private key and addresses. All are generated by the ledger nano. Which means, if truly you have your bitcoin on ledger nano, the Bitcoin is still then on your ledger nano wallet, and can be easily accessed using electrum wallet.
I have the public key which shows that the BTC is still there. It's a Legacy (P2PKH) wallet and according to Magnum the derivation path is: 44'/0'/0'/0/0
The derivation path suppose to be m/44'/0'/0'/0/0, and I suspect that you are mixing things up. Is the Bitcoin truly on ledger nano or magnum wallet? I am suspecting you created a magnum wallet, generated seed phrase and the keys. You need to let us know where you generated the seed phrase. Is it on magnum wallet or you used magnum wallet as an extension to ledger nano?
I've tried connecting my Ledger to Electrum but I'm not seeing the address there. Was wondering if anyone else has run into this issue or has any ideas on how I can access my BTC?
Like I have commented before, likely you generated seed phrase on magnum wallet, not using magnum wallet as an extension of ledger nano. If so, all you will need to do is to first check if the seed phrase from magnum wallet is BIP39 using iamcoleman tool, if it support it, then you can import the seed phrase on electrum desktop wallet, it should work successfully. Electrum wallet also support importing master private key and private key. If you truly used it as a wallet extension of ledger nano, then it will work on electrum and your bitcoin will be on the ledger nano balance.
For bitcoin storage, I will not suggest you such wallet, using reputed ones like electrum and mycelium are better. Even as an extension of hardware wallet, I will recommend electrum desktop wallet.