To achieve this goal, the best option is to increase the size of the VM instance. The Standard_D2_v3 virtual machine size has a maximum network bandwidth of 1 Gbps, so increasing the size of the VM instance to a higher tier, such as Standard_D8_v3 or higher, will provide more network bandwidth and improve the application's performance.
Option A, requesting an increase in networking quotas, may not be sufficient to achieve the required network bandwidth.
Option C, disabling the Azure Firewall and implementing network security groups, may not have a significant impact on the network bandwidth.
Option D, moving two of the servers behind a separate load balancer and configuring round-robin routing in Traffic Manager, may improve availability and performance but will not increase the network bandwidth.
Source: [1] https://docs.microsoft.com/en-us/azure/virtual-machines/windows/sizes-general [2] https://docs.microsoft.com/en-us/azure/virtual-network/designing-hub-spoke-topologies#optimize-data-transfer-between-hub-and-spoke-vnets

To troubleshoot the cause for the VPN disconnections between VNetGW1 and the partner site, you should verify that the partner's VPN device is configured for one VPN tunnel per subnet pair.

Two possible causes of the issue where a function returns the error "Azure Function Runtime is unreachable" are: C. The storage account application settings were deleted. E. The storage account for the function was deleted.
According to Microsoft, this issue occurs when the Functions runtime can't start. The most common reason for this is that the function app has lost access to its storage account. If that account is deleted or if the storage account application settings were deleted, your functions won't work
https://learn.microsoft.com/en-us/azure/azure-functions/functions-recover-storage-account