Which router will perfectly cover 70kv and How to make 2 internet through a router?

Question

WiFi

Now there is a Zyxel Keenetic Omni II router
(4x1000 Mbps, 802.11 2.4 GHz, Wi-Fi 300 Mbps),
but for some reason, after I moved it from the corridor to the hall, it began to cover the entire apartment rather poorly.
and secondly, for some reason, it makes a little noise, as if ultrasound is distributing.
I need
1. Silent in general
2. To cover the whole apartment well.
3. In order to be able to set up uninterrupted Internet, I want to insert a 4g modem into it along with the main wired Internet (also advise)
4. It is necessary that the router can switch automatically from the main Internet to the 4g modem if the main wired Internet is not available. (is it possible at all?) (otherwise the Internet is often turned off lately, but I need everything to be uninterrupted)
I looked after myself ASUS RT-AC58U
www.dns-shop.ru/product/128848e76fe13330/marsrutiz...
will it suit all my tasks?