This solution might be over the top for most people, but I enjoy thought experiments like this.
I use my phone in multiple areas, and in some areas, only Verizon has good coverage. In another, only T-Mobile has good coverage. If poor coverage is limited to a building with Internet service that you can connect to, then a microcell can be used to fix this. AT&T discontinued their microcell because Wi-Fi Calling makes this device unnecessary for most people. If the signal is weak, a signal booster might work. For me, AT&T covers both areas, but very poorly with dead spots. A portable signal booster might work, but probably not in some spots. If I travel to rural areas exclusively covered by different carriers, a dual SIM phone will let me receive service from 2 different carriers on a single phone, but I will have a different phone number for each service.
A Wi-Fi connection-only SIP/VoIP phone connected to mobile hotspots solves this. A VoIP/SIP service with a Bring Your Own Device plan will let me send and receive calls over the Internet via Wi-Fi. To use this outside of normal Wi-Fi coverage, I would get a Alcatel LINKZONE for T-Mobile and a Verizon Ellipsis Jetpack for Verizon. I can remove the cell modem from the Librem 5. Not using the Librem 5’s cell modem will increase its battery life, but now I have a second device to charge, the mobile hotspot. I am guessing that the battery lasts for about 8 hours for their intended purpose, but for a cell phone that idles most of the time, I hope that it would last a little longer. While I would have to carry 2 devices around, only the weaker Wi-Fi radio will be next to my brain, while the cell modem will be blasting away in a pocket or on a desk. With any luck, the Wi-Fi chips will automatically turn down the transmit strength given how close the receivers are. Commercial grade Wi-Fi devices usually let you lower the transmit power, consumer devices, not so much.
Operating 2 hotspot only plans is expensive, but I can connect it to my laptop and tablet and not get charged for another device or have to pay for tethering. So I am getting 3 devices connected to cell service for this. Also, I can use it with my desktop (via a USB Wi-Fi adapter) if my wire based (fiber, cable, DSL) Internet service goes out. The starting plans do not include a lot of data, but most of what I do when I am not on Wi-Fi is simple browsing and e-mail. I tend to not watch video when I am on cell service, mostly because I do not want to be too disconnected with my surroundings when I am not in my own space.
Another down side to this is that there is no way to send or receive texts from the phone. The hotspots can receive text messages and can display them in their web administration interface, but at least one of them is limited to receiving text messages from the carrier, so it cannot be used for receiving texts from other people, in theory. Also, one of them limits the number of texts stored to a very small amount. Using the hotspots for text messaging would have the same problems as having 2 phone numbers on different carriers. An SMS gateway with an e-mail client can send messages. If you have an old iPhone or Android phone service, then you can configure it to forward texts to an e-mail account. TextPort or TextMagic lets you send and receive SMS from a web browser. SMS should be treated as an unencrypted messaging service. Avoid it if possible, but it is a least common denominator for people with cell phones.