root@OpenWrt:~# ip -4 route
default via 192.168.5.1 dev pptp-vpn
46.147.242.79 via 192.168.1.1 dev wan
192.168.0.0/24 via 192.168.5.1 dev pptp-vpn
192.168.1.0/24 dev wan scope link src 192.168.1.2
192.168.5.1 dev pptp-vpn scope link src 192.168.5.2
192.168.8.0/24 dev br-lan scope link src 192.168.8.1
192.168.9.0/24 via 192.168.5.1 dev pptp-vpn
192.168.10.0/24 via 192.168.5.1 dev pptp-vpn
192.168.11.0/24 via 192.168.5.1 dev pptp-vpn
I only need a VPN server to access other local networks (so that computers from different networks can see each other). Each computer in each office should access the internet through its own provider.
Don't use PPTP, consider other solutions like WireGuard.
Use the configuration option I suggested earlier, otherwise look for the corresponding checkbox in Luci - "Use default gateway" in Interface, Advanced settings.