A code P1811 indicates that one of the transmission shift adaptives has reached it's maximum limit. In other words, the transmission is taking much longer than it should to complete a shift, and the computer has reached the extent of it's ability to compensate for the problem.
This type of fault can be caused my dozens of different internal transmission problems, such as an internal hydraulic pressure leak, valve body related problems, low fluid line pressure, or other internal problems.
This is not really the type of fault that you can accurately diagnose nor repair yourself at home; this particular repair is one to take to a transmission specialist. Some special test equipment is required for diagnosis and repair, and it is probable that some degree of transmission disassembly will be required to correct this type of fault.