Hi@ThePureVessel45,
Welcome to our community.
May I know if your network topology diagram is as follows:
ISP modem—router )))((( client devices
(Note: — stands for wired connection,)))((( stands for wireless connection )
Is the solar panel inverter connected to the router's network wirelessly or wired? Can you provide the specific model of the solar panel inverter?
At present, you have tested that the network speed can be restored by removing the solar panel inverter, so the problem is likely to be with the solar panel inverter. It may be a power quality problem caused by the solar panel inverter, or it may be electromagnetic radiation interference.
I suggest you contact the solar panel inverter support to confirm whether there are any settings that affect the network speed?