Network with two gateways and a managed switch

I am no network engineer, but I have to find a solution to a networking problem for work. Any help or advice would be of much appreciation .

I have the following network topology:

https://drive.google.com/file/d/1ISKdIaGFfzbyY9y9hHH3RfPgCWo8iZsU/view?usp=sharing

PC1 has a GSM module connected to it Via USB as shown. This GSM acts as the main gateway for the entire system providing internet access to both PC1 and PC2.

To have internet access on PC2, IP routing rules were setup on PC1. Both PC1 and PC2 are configured with static IPs and are connected through a managed switch. This network works fine.

Now, I have an extra requirement and that is the possibility to provide internet access to the system via an external port, by connecting an Ethernet connection to the switch. The reason is that in case there is a need for a big software update, users can connect the system to their private network and download the update without using the GSM module.

The problem falls here, as the private network will most likely have DHCP to assign IP addresses to connected devices. So, I have two questions:

1- Is it possible to meet the internet requirements with current hardware setup, knowing that a part of the network must have static IPs? and how can this be achieved?

2- If its not possible with only a switch, what can be the most appropriate solution? I am assuming in this case a dual WAN router?

Thank you for your help :)

There are several possible solutions, but the easiest would be to add a second Ethernet interface (for example a USB Ethernet adapter) to PC1 and use that to connect to the home router. You can then configure that additional interface for DHCP and it will get a default gateway from the home router when connected. Set the GSM interface to offline for the maintenance work and PC1 will use the newly acquired route to the home router for Internet access. You won't have to change anything on PC2.