Here is a recipe that worked for me to get TAN2Go App running and receiving TANs + activating Visa secure on a Huawei P8 Lite with LineageOS 14.1. with using Island.
It will only work, if you mobile number is already registered in DKB, because you have to receive an SMS.
At some point you need to open an http link from the SMS via TAN2Go app on island. Since the SMS app did not work on island for me, I used a workaround using the contacts app.
Some steps in the beginning, especially the factory reset might not be necessary, but this is what I did.
0. Lineageos Factory reset (TWRP wipe)
1. Install latest Magisk / Magisk Manager
2. In Magisk Manager Settings, enable Magisk Hide, repackage the Magisk Manager Application
3. Install DKB Banking App / Tan2Go / Island using Google Play Store
4. Open repackaged Magisk Manager, hide DKB Banking App / Tan2Go / Island / Google Play Store
5. Restart the Phone
6. Open and configure Island
7. Install / Clone Tan2Go app and DKB banking app from mainland to island
8. Install / Clone the contacts app from mainland to island. You may now open it in island to check if it works and that you can create a contact.
9. Deinstall the Tan2Go and DKB banking app app from your phone (the "mainland" versions!)
10. Open Tan2Go "island" version, set new Password for the App.
If this step does not work, using Island to bypass the protection may not work on your phone.
You could try the Magisk Canary build, wait for a new Magisk version, or try a factory reset like I did.
11. Tap on "Tan2Go erneut verknüpfen" (NOT the QR code, which worked for me only once), proceed to online banking. This will open DKB Banking App.
The next steps are time-critical, because your session may expire. So read the steps first.
12. Login with your banking credentials
13. In the FAQ section open drop-down menu "Sie können derzeit keine TANs mit der DKB-TAN2go-App generieren?" and click the Link inside the text: "TAN2Go-Verwaltung"
14. In the connected devices section, choose the device that you are currently holding in your hands and click "Password vergessen"
The next steps are especially time-critical, but unfortunately may take some time to execute
Once you request the SMS, you got a time window that the SMS code is valid.
15. On the next page click button "Neu verknüpfen per SMS" and then "SMS anfordern", receive the SMS, copy the link (or the full text if not possible)
16. Open contacts inside Island, add a new contact, chose contact name, paste the link from SMS to "Website" section of the contact, save contact
17. On the generated contact, click the website link, chose open with Tan2Go app
18. (I had to re-set the Tan2Go password here for some reason)
19. Copy the code "Freischaltcode"
20. Go back to the DKB Banking App in mainland and enter the code. If you were to slow, repeat from step 10.
21. You may now enable Visa secure in DKB Banking App.