You're right! I'm now using this ONT as an access point and it doesn't have any issue with Ethernet ports. Maybe Huawei can fix it with a new firmware fix. I didn't want to, but at least I can use it as AP.
I suggest you check the fibre conection quality, see if the optical power intensity is among the best working range. Normally when it's close to the threshold, the connection quality is of no good.