In steam deck settings, there’s an option for external monitor safe made. This limits resolution/refresh rate/hdr to improve compatibility. You can also try manually limiting external monitor resolution.

I know I had an issue where an slightly older TV I had started rejecting signal inputs if the resolution was too high or hdr was on. Originally I thought my deck’s video output was broken, but I tried a 4k chromecast and the TV couldn’t display it either (despite being a TV that had previously supported 4k and HDR). Limiting to 1080p with HDR off made everything work again.