If water is available and usable, then performing the ablution with the water is a prerequisite and condition for Salat. Tayammum is only for exceptional condition where water is not available or one is not able to perform ablution with water for medical reasons etc. Just because Salat in congregation has started, one cannot decide to leave ablution with water and just perform Tayammum.

In Sahih Muslim it is recorded that the Holy Prophet^(sa) said:

لاَ تُقْبَلُ صَلاَةٌ بِغَيْرِ طُهُورٍ

“No Salat is accepted without Wuzu’ (purification)” (Sahih Muslim, The Book of Purification)