SOLVED Telem only working on re-plug not on boot

I have a genuine ph 2.4.8 on a quad. Up till now it has been using an RFD900 telem unit without issue at baud 57 on telem1.

I want to swap the RFD unit for a skydrone 4G link which is a Rpi connected to telem2 at baud 115.

I have an identical pixhawk unit which I have been testing on and everything works perfectly (same firmware 3.5.3). Connected via telem2 I can get a connection in mission planner and everything is great.

When I try and move the hardware across to the “production” aircraft things dont work properly.

If I power up the aircraft with the telem cable connected to telem2 I can’t connect to it over the 4G link. However, whilst powered up if I swap the cable to telem1 I can get a connection… If I cycle the power with the cable in telem1 I cant connect and have to hot-swap it over to telem2 to get a connection… This is most odd… Any idea why I can only get a connection if I hot swap after boot? The other “test” pixhawk doesnt suffer the same issue… Is there an odd param somewhere I am missing?

It appears to now work properly by changing
BRD_SER2_RTSCTS to 0 For some reason the auto setting does not work on this pixhawk. The test unit does work fine.