Is this pinging from Wlan side (i.e. from CPE to 433 AP) and not the Ethernet if yes I have encountered this a number of times and it was caused by water getting into RJ45 connector and corroding and eventually it loses connectivity.
How are you powering the RB433ah? PoE or power jack? What voltage power supply are you using?
add: When is the failure happening? Is it late night or very early morning? The reason I ask is this from the user manual:
J9 power jack:
10..28V DC (cut-off voltage - 30V)
Power over Ethernet (PoE) on the J601 LAN1 Ethernet port:
10..28V DC (18..28 V suggested; cut-off voltage - 30V) non-standard PoE powering support
Overvoltage protection starts from about 28.1V-29V ±delta (depends on some part physical options, works up to 60V), so the board will not be damaged if connected to a 48 or 60 V power line.
A 24 volt unregulated power supply will easily reach 30 volts at night when the line voltage is highest.
For the same reason, I use 12v solar, even tho the manual says it is ok to use 24v solar. My solar charger compensates for ambient temperature, and with a 24v system in the summer I am ok (max charge voltage around 27.5v), but in the winter it is not (max charge around 30v).
also many rb433 boards with green cap syndrome are still in circulation. I received a new RB433 recently from my supplier and it had the dreaded green caps. I just replaced them, cheaper than sending it back.
How long is that CAT5 cable run from the PoE injector to the RB433AH? You will get some voltage drop, but a drop of 6 volts would indicate a long section of CAT5, bad connectors, or a faulty PoE injector/power supply.