@MikeyG23 

You have two basic choices....Load Balance or Failover.  The former uses both connections (active-active), the latter will use 1 connection unless it fails, and the switch to the other.  It doesn't matter which is connected where.

If the old connection is so terrible, I'd question why you want to use it at all.  It would just mean that any connections load-balanced on to it would suffer the low bandwidth/low reliability.

If it was me, I'd set it up for failover, ie use shiny new connection unless it fails, then flop over to old connection.  There's a tick button in the Failover to force the router to switch back to the 'primary' connection if it comes back online...you'll want to tick this for sure!

  @MikeyG23 

Looks good!