Yeah, it's always warm.
So is mine.
But it only matters if it gets significantly hotter than normal. By 10-20 degrees.
"Warm" is unlikely to be an issue.
I don't think modem activity would make any difference to the temperature anyway even if it was running flat out.
The heat from a router is generally the routing SoC, whereas all this VDSL work will be done in the modem component with its own CPU/DSP for that stuff. If it was struggling the sync would drop, that's it.