Dell Inspiron 15 3530 — Orange Power LED and No Boot

Hi, hoping someone might be able to help or offer advice regarding a frustrating intermittent boot issue I’ve been having with my 18 month old Dell Inspiron 15 3530 

On startup, when I press the power button, instead of powering on normally, the charging indicator LED turns solid amber (orange) while the power button is held down.

• The laptop stays completely dead — no display, no Dell logo, no POST, no fans.
• Releasing the power button turns off the LED, and nothing happens.
• This occurs both when connected to AC power, on battery alone, or both
• It occured yesterday for the first time after a restart, and eventually after several attempts I was able to get it back up again and it ran like nothing happened
• Today however, it shut down unexpectedly, and despite trying the same things I did yesterday to get it to work, it has not turned on since, only the orange light

Additional behaviour:

• Occasionally, when the system does eventually boot (after several attempts or reseating the battery), I get a message saying “BIOS has been reset” and need to reconfigure the date/time in BIOS, though I am aware this may have been due to disconnecting and reconnecting the battery
• Once it boots, the system runs fine with no crashes or errors

Things I’ve tried:

• Multiple power reset attempts (disconnect AC adapter and internal battery, hold down power button for 30-60 seconds).
• Booting with AC adapter only (battery disconnected) and only the battery — yesterday sometimes these worked, today I have had no success with either 
• Running Dell ePSA diagnostics when it boots — all tests pass.
• BIOS and drivers fully up to date via Dell SupportAssist.

Notes:

• AC adapter indicator light is steady when plugged in.
• There’s no diagnostic blink code — just the solid amber light while the button’s held.
• No known liquid spills or major physical damage, only a dent in the bottom right corner from over a year ago which caused no problems at the time or since so it is unlikely to be related

Questions:

• What does a solid amber charging LED while holding the power button indicate on this model?
• Are there any additional steps or diagnostics I can run, or should I be considering a battery or motherboard issue?

Would really appreciate any advice or insight — thank you!