Did you label the wires on the old thermostat locations first? Colors don’t always 100% line up to where you think they go.
Y is AC/Cooling
G is for the FAN
W is for heat
R is 24V AC power. Many times the 2 Reds RH and RC are connected together,
C is Common
Common as in ground!!! You need to have C connected as the thermostat needs a ground connection for power. It’s getting the 24V AC which it passes through with the relays to turn things on off as those things are grounded. It also uses that to power it’s self, but it also needs its own ground. On my system, I didn’t have a C wire. I did have a couple wires tucked up out of the way and I used one of those for the C wire, and connected theother end of that wire to the C terminal on my Heater.
If you don’t have the C wire, then you have to use the adapter. But it would be better if you didn’t have to. I did the same thing at a friend’s house. Heater in the attic. Pulled the panel, connected up a wire for C and did the same at the thermostat end, Works great!!! No adapter needed.
Never trust what color wire is used where. Which is a good reason to take a picture or 2 of how your wires are connected first just in case. I had of course more than the 1 wire for C. It’s common, and so connected more than 1 wire is just fine if there is a wire in C already on the heater. Common as in ground!!! So at minimum, you need the R and the C to power up the thermostat. The G turns on and off the fan. Good to control independently so say the heater has been running and shuts off, there is still heat in the system, and will continue to run the fan and blow out the heat still in the system.
Of course you have Yellow and White as that is your AC, and so you need to trigger your compressor and fan outside. I assume there is a connection for that on your main board with the 2 wires, with just yellow going out to the thermostat. Of course you have W for heat. There are relys on the thermostat to turn the flow of power (24V AC) to turn on heat or AC or the FAN. A relay for each of those. Those all have common/ground on their own also at the main board.
I’m trying to make it as simple as possible. That in general, it’s not magic on how it works. If you know the basics you can generally figure out what you did wrong and fix it. Take your time, step by step look things over and you may just go OPS!!!
Make sure the thermostat wire colors on the main power board at the heater match the wire color locations on your thermostat plate. If you happen to have a extra wire to use for C, by all means use it. It really is pretty simple. Connect both ends to C!!! My Termostate wires are right next to my heater. So If I needed to, I could just go to home Depot and get new Thermostat wire that had enough wires in it for my needs so I could add a C. Luckly like I said, there was a couple spares tucked up behind. But if not, and it’s not hard to pull a new thermostat wire bundle, that is another option.
I was having issues with my Main Board on the heater. The thermostat would turn off, but the heater would keep on running. I could hear it clicking, but not shutting off like it was supposed to. I ended up ordering a new board and swapping them out. It now works as it should. That is a whole different topic.
I hope I was a little bit of a help.