I would treat the two elements completely separately as it will simplify the broadband provision and avoid potential mistakes.
1. Arrange the installation of a landline (if you don't have one). The number on this line is irrelevant as you won't be using it for calls.
2. Order a broadband service on the new line
These steps can be combined if you're happy for the ISP to also provide the line
3. Port your VOIP number to a new VOIP provider
4. Cancel your WISP contract
Nice and straightforward, minimal risk of losing the number and keeps your number and broadand separate to ensure future flexibility.
If you're already used to using VOIP I can see absolutely no benefit in going back to POTS telephony so there's no need to link the services together.
Edited by ferretuk (Mon 04-Dec-17 18:36:00)