The two issues in the wireless realm are the router and the NIC (wireless interface) on the tablet. Not all routers are created equally, some just periodically drop connections, some have a limited lifespan.
The few android tablets I have played with were intermittently flaky with my router but my iPad is rock solid ( you can fault Apple for a lot of thing but quality is generally not one of them.)
I would start with another router. Borrow one from your house and check it for reliability. Since all the tablets have a problem with connecting ( that is correct, right?), I would suspect the router first.
Most of the cable I have seen at the stores is 5e for the standard stuff. It is better than 5 , and 6 has somewhat better throughput. You likely have 5e.
Another option, depending on your layout of your office would be to get a range extender. This basically takes the initial signal and boosts it up and retransmits. Some routers can be shifted into the mode. The problem is, what if you get one and it doesn't help? Sure, you can return it but the time and energy may not be worth it. The flip side is that you may be able to maintain wireless.
Hard wired is a fair bit simpler once it is in place. Not as flexible, but more reliable.
