USRP channel frequency change from serial port data

Dear Community,

I want to control the channel frequency in the RF options in the USRP sink block on GNU Radio Companion from the float data coming from serial port through a custom python block and the data coming is showing in the GRC environment in bar sink as float values and I am trying to save these float values in the variable 'frequency' by using another python block, and when I write the same variable 'frequency' in place of USRP channel frequency option, it's not recognizing this variable. I have tried message pair to variable block, but it is still not recognizing frequency variable in USRP sink block.

Is there any way to control the USRP channel frequency through the data coming from serial port in real time. Kindly help me in this regard.

Best regards

Muhammad Anas