The Bangla Unicode font (Solaimanlipi) is one of most complicated font. Basically it have some complex combination e.g. ক্ষ ঙ্ক ঙ্গ জ্ঞ ঞ্চ ঞ্ছ ঞ্জ ত্ত ষ্ণ হ্ম ণ্ড কো etc.
Please help me to get a solution.
You need to set your font like this.
$fontname = $pdf->addTTFfont('/path-to-font/yourfont.ttf', 'TrueTypeUnicode', '', 32);
after that you can set your font like this...
$pdf->SetFont('times', 'BI', 20, '', 'false');
TCPDF
documentation gives you clear explanation for this. See here