I don't know the TX51 specifically, but across the board (especially with Boscam) transmitters are generally more powerful on the first channels, and weaker on the higher channels... so go with channel 1 -3.
Are you sure you've got the BP receiver tuned to the exact same group/channel to match frequencies with the transmitter? like have you manually set the channel? You should have a similar chart with channels/frequencies for the BP... make sure you match exactly.. often when people use the "Scan" feature it doesn't pick the exact right channel, it'll pick a nearby channel
many people fall into the trap of thinking that transmitter wattage is the main factor in FPV performance, when it's actually your antenna quality. 1000mW transmitter with **** antennas is going to perform way less than a 400mW transmitter with premium antennas. I'm running a TS353 and I've personally been out to 1800m with rock solid FPV no different than when it is 10m away... but I've got the FPVLR pentalobe on the Tx and the 4.25 helix on my receiver.
environment is also a factor, urban settings with lots of interference will adversely affect your performance vs being out in the country with no interference