This “one month at a time” approach works only if the Govt, by the end of June, agrees a long extension (the outer limit of what is needed, up to 2 years.
The key provision is Art 132(1) of the Withdrawal Agreement. 
Important points. 1. It’s a once-only extension. No provision for further extension.
2. The “up to 1 or 2 years” phrase is dreadful drafting. As I explained here.
But I “think” it would allow an extension of the form “until 31/12/2022 [or 30/6/2022 or 31/12/2021] but earlier if so decided by a FTA agreed between the EU and UK before that date”.
That would create some difficulties in negotiating money (see Art 132(3)). But not insuperable ones.
3. What “can’t” be done is to keep negotiating month by month (or 6 month by 6 month extensions). You have to commit to a fixed period, with any early ending of that period subject to mutual agreement.
