access client side lan

[B-Man]

I have a Vpn server running on my home router and have a PI at my parents house that connects to the vpn server as a client.
I can remote desktop into the pi but it is so slow.
Is there a way i can connect from my home pc to the client lan ?
I normally need to access to the routers to change or set something up for them and that can be done but painfully slow through the Pi but now i need to access their solar inverter GUI. and while a cna do that for some reason the Pi web browser shows it as english when loading but then displays it as chinese once fully loaded. so i can't see what i need to edit.

[Pippin]

I normally need to access to the routers to change or set something up for them and that can be done but painfully slow through the Pi

So, from your home PC you can access your parents LAN...
From what you describe it is working, LAN to LAN.

With regards to speed, that can depend on several things.
Your up/download speed
Your parents up/download speed
Pi not that powerful
and some more.

See solar inverters manual on how to change language.