TRIP.COM — Flight Change request Denied though Airline said it's possible

I have booked a flight for Gulf Air through Trip.com for Mumbai-Frankfurt via Bahrain which further I need to travel to UK. Due to Airport transit Visa requirement in Germany, had requested Trip.com to change my flight from Bahrain-London rather than Frankfurt which they said it is possible and price difference is to be paid. After speaking with at least 10-15 Specialists from their US customer care they kept me waiting for the payment for the price difference and finally said their policy doesn't allow change of destination. Gulf Air said it is possible but to be done by Trip.com as tickets booked through them. They wasted more then 30 hours and never mentioned that it is not possible. Highly Frustrated and Worst website. Will never book again with them, Also, for cancelation you will only get the taxes back which is hardly 5% of the ticket book price.