when can i serve section 21

Rent Arrears

My Tenant is 6 months into a 12 month AST, which has a 6 month break clause in anyway. He has 5 months rent arrears.
I am confused when I can serve Section 21 notice because of the governments clauses due to Covid. i have an estate agent managing the property, and all the compliances have been met i.e EPC, deposit scheme etc.
MY AST says I give 2 months under normal circumstances. I understand this is 3 months due to Covid. But I am confused now it says there is an eviction ban for 6 months.
Also, online it says that after 6 months of rent arrears I can give 4 weeks notice.
So, I am unsure whether I can issue notice, can I issue notice now, or is it better to wait a month more to make it 6 months arrears and then issue 4 weeks notice ?
Also, form 6a is new to me, do I need to do this too.
Tenant applied for property using own job, wage etc. 1 month afterwards has applied for universal credit/housing benefit and become registered disability.He also has a guarantor but have said they cannot pay my rent. Will any of this make a difference to me issuing Section 21?.