From Kerala reach Guwahati by train . from there travel to Bomdila (guwahat-bomdila approx 12 hr journey )
. From Bomdila you will get bus or shared taxis to Tawang .
PS: You need to take ILP (Inner Line Permit )to travel in Arunachal Pradesh . ILP can be taken via online ( https://arunachalilp.com) or Offline from the ILP facilitation centers present at Naharlagun Railway Station, Gumto Railway Station, and Guwahati LGBI airport. You can also visit Guwahati Asom Paryatan Bhawan and or DC’s office in selected cities ..