Hi, from Dhaka to Shanghai there is only connecting flights with transfers at Singapore available on that day.
Below is the flight schedule for your reference:
Dhaka – Singapore SQ435 0055/0555
Singapore- Shanghai Pudong SQ828 0805/1325

From Shanghai to Singapore there are several flights available each day, here is the shortest flight time schedule: Shanghai Pudong – Singapore MU545 1605/2050

From Shanghai to Dhaka, there are connecting flights with transfers at Kunming, Singapore or Kuala Lumpur available on Saturday. And transferring at Singapore will take shorter time:
Shanghai Pudong – Singapore SQ831 1430/1945
Singapore – Dhaka SQ436 2040/2340

From Shenzhen to Dhaka, there are connecting flights with transfers at Kunming, Kuala Lumpur, Bangkok or Beijing available on Saturday. Below is the flight schedule with transfers at Lumpur.
Shenzhen- Kuala Lumpur AK81 1125/1515
Kuala Lumpur- Dhaka MH196 2220/0110

For more, you can consult the booking agency or submit the online booking form in the website.