Im also use the same road from India to dhak and Bangladesh I used Katmandu
From Katmandu you have to take bus to Kakorvita border of India (14 hours journey). From Kakorvita, you have to come Burimari-Chengrbndha border (Bangladesh-India border) by bus or taxi (4-5 hours journey. After crossing the border, you will find some buses to Dhaka (12 hours journey). Or you can discover northern areas of Bangladesh (Dinajpur, Rajshahi, Bogra, Natore, Rangpur) & come Dhaka gradually.