How to organize access to the 1C server from two different networks with two routers?

Question

network hardware

Welcome all. I will be very grateful for advice, recommendations and practical instructions on the problem that I have.
I work in a carpentry shop as an accountant and part-time system administrator. I studied everything myself, so basic gaps in knowledge are possible.
Initial data:
There is a small network of 10 computers, 2 servers (1C server, video surveillance server), 2 switches (24 port Cisco, Rubitech), 2 routers (Zyxel Keenetic II), 1 DVR with analog video surveillance cameras and 8 IP cameras. All this is located in a workshop in an industrial area where there are very big problems with connecting to the Internet. Previously, the company did not have any servers or IP cameras, and the Internet was supplied via an ADSL modem and was extremely unstable and slow. The authorities gave the task to put everything in order, to organize the work of 1C accounting and video surveillance systems (and to make everything cheaper as always).
And so began the saga of setting up equipment and searching for the best implementation of all this. The main problem was on the Internet, in particular, the problem of the lack of an external IP address (it is necessary to implement access to the network from the outside and the operation of video surveillance), in my case it was possible to get a more or less acceptable Internet only through 3G, 4G providers. For this, a Zyxcel Keenetik II and a USB 3G modem for it were purchased and configured. After settings, trial and error, it was found out that providers who have real unlimited do not give white IPs and cut the VPN channel, while other operators allow you to connect a dedicated IP service for adequate money, but do not give unlimited Internet. Another problem is that in addition to 10 computers, there are a lot of employees with phones, tablets, etc. which consume a lot of traffic.
In the end, I came to the conclusion that we need two routers with two different modems. One (let's call router A) with limited internet and white IP for servers, and the other (let's call B) with gray IP for everyone else.
Question: How to set it up? The problem is that ordinary users must have access to the Internet from router "B" and to server 1C, and the same server must have access from router "A" to connect to it from the outside, well, access to the main network through router "A" too would not interfere. How to implement it?

Answer 1

In the end, I came to the conclusion that we need two routers with two different modems. One (let's call router A) with limited internet and white IP for servers, and the other (let's call B) with gray IP for everyone else.

different "default" routes for servers (router A) and for regular workstations (router B).

Answer 2

Here I drew a diagram. Those. in the server gateway you need to set the IP of router A?