It is embarrassing to mention, but my cellular reception all across the Iberian Peninsula is far better, and at higher speeds, than here at home in Palm Beach County, Florida. There are technical, economic, and political reasons for this disconnect, but it exists.
In general, European and Asian countries, late to providing modern, nationwide phone networks (like, well after WWII) opted for cellular towers and service over old-fashioned hard-wired land lines. While most of these countries had earlier, basic, land-line networks, by the 1980s when telecommunication demand soared, most countries outside the US went for the advantages of wireless communication based on well placed cellular antenna towers.
Communication service providers in the US remained focused on hard-wired solutions for several decades, only until recently (last 10 - 15 years) adopting the full wireless business model. As a result, we are usually on the tail end of adopting new wireless technologies.
Typically, you will always have at least 3G coverage, with at least two signal bars, depending on phone. In large towns and cities in both Spain and Portugal, I typically get near maximum signal strength with full LTE coverage.
The unique limiting factor in Portugal or any old European country with lots of really old stone structures, like churches, municipal buildings, hotels and in general is signal propagation. Inside these structures, your signal strength will be reduced to zero or near zero.
In fact, my home in Florida is made of steel reinforced, concrete blocks (hurricane resistant construction codes) with stucco on the outside and a concrete Spanish "S" tile roof. This mass reduces my cellular signals to ZIP. If it were not for the ability to use my t-Mobile iPhone over Wi-Fi, I would have to go outside to receive or make a phone call. Cellular data is simply not possible without the Wi-Fi.
Also, when I work at the new Pilgrim Office (rua Carretas) in the summer this is an issue. You will see staff and pilgrims realize that the only place they can get a decent data or voice cellular signal is to go out on the western terrace, adjacent to the Pilgrim Office. So, regardless of where you are, being in a massive stone or concrete building will pose issues.
Also, I cannot recall being in a cellular "dead zone" anywhere in either Portugal or Spain. I am sure these locations exist. But, I have not yet encountered one (except in massive stone buildings.).
I hope this helps.