1. The spec of Asus RT-BE88U says that the 10G SFP+ port can only be WAN, I tend to believe it can also be WAN/LAN. Please check.
https://www.asus.com/sg/networking-iot-servers/wifi-routers/asus-gaming-routers/rt-be88u/techspec/
I/O Ports:
1 x 10Gbps for WAN/LAN,
1 x 10Gbps SFP+ for WAN,
1 x 2.5Gbps for WAN/LAN,
3 x 2.5 Gbps for LAN,
4 x RJ45 10/100/1000Mbps for LAN,
USB 3.2 Gen1 x1
2. If the SFP+ port can only be WAN port, then you will have to use it as WAN port and then the 10G RJ45 port as LAN port. Either way you will get hot stuff. In that case, just use a fan to cool down if worried.
1) Using the normal ONT (bigger FIbreHome bridged ONR, or smaler Huawei ONT without voice fuction), then on the WAN side you may need to have a SFP+ to 10G RJ45 copper module which can be hot. There are low temperature modules though (about S$60, cheaper than S$99). So this may still be a better option if you are worried about heat.
2) Using the Huawei SFP+ ONU stick which is also hot.
3. If the SFP+ port can be used as WAN/LAN port, then you may want to use the 10G RJ45 port as WAN and the SFP+ port as LAN. In that case, you can only use the normal ONT.
On the LAN side, you can use either the low temperature SFP+ 10G RJ45 module, or you can use a DAC cable to connect to a mixed 10G SFP+ and 10G RJ45 switch, if you are willing to add a switch.
4. More info --> please refer to the following thread, about the switch and SFP+ modules.
https://forums.hardwarezone.com.sg/...-port-switch-and-vq-xgs-pon-onu-stick.7047645