There are two important considerations before approaching the firmware issue: first, do you have an all-metal, direct-drive extruder? If you don't, your Bowden tube will melt where it enters the hot end and lead to clogs before you ever reach 300 °C (the same may be true of other plastic parts, if present). Second: is your heater cartridge rated for high enough power to actually heat the hot end that hot? There are things like silicone socks that can help here, by keeping the heat break and print cooling fan airflow off the actual hot end -- but this may not even be possible with the stock cartridge for an Ender 5 Plus.
Assuming you've already dealt with those potential issues (and given you can print at this temperature via slicer settings, it seems you have), it seems likely that the Marlin firmware has hard coded limits in the adjustment modules or the LCD input module that prevent manually setting the temperature to a range that is none the less within the reach of the actual control PID. The only solution for this is likely to be analyzing the source code for the input and manual control modules to find and increase that limit (it may exist in more than one location, and they may not all have a value of 260 °C).
Obviously, you'll want to store an unmodified (or at least a known-working) copy of the firmware before you delve into what might be poorly documented or undocumented parts of the code, but the beauty of open source is that at least this is possible.