If you have a wallet with 18 seed phrases, it may not be supported by some popular mobile wallets, but there are alternatives that support 18 words, such as Electrum or Mycelium also you can try Atomic Wallet or Ledger Live, which both support BIP44 standard
Atomic wallet only supports 12 words seed phrase. Also close source.
If I remember correctly several hardware wallets are supporting 12, 18 and 24 seed words and Coinomi software wallet was also supporting 18 seed words.
I would first try using iancoleman website but remember to do it only offline, not connected to internet.
Coinomi support 12, 15, 18, 21 and 24 words seed phrases, but just that it is close source, when there are open source alternatives, I do not think it is necessary to use it.