You are for sure using:

https://github.com/ftomassetti/backtrader-oandav20

Without looking at the code I would dare to say that the store doesn't re-open the connection simply because you update a couple of attribute members whilst trading.

You will have to ask the author(s) to add a method to do such a thing, something like this from within the strategy

self.broker.update_token(my_new_token)

and have this method creating a new connection to Oanda which will replace the existing one.