Cell phone coverage in Hokkaido

Hi, I will be travelling to Tokyo and the Hokkaido (going to Hakodate, Sapporo and Asahikawa). Any recommendations about which carriers are best for these? I imagine that any carrier will be good for coverage in Sapporo and Tokyo, but am interested in recommendations for good coverage in other parts of Hokkaido. Thanks

Hakodate and Asahikawa are cities with a population of 340000 and 265000 respectively . There should be no problem with mobile phone coverage in either

Cell coverage in places where people actually live, work and visit will be good - that is pretty normal.
Going somewhere where there is nobody for miles around the coverage will be patchy - i.e. if you go hiking in the mountains or staying at a small/remote place.

Docomo.

Last November we purchased a Simpower data only cards for Hokkaido ( travelled as far as Sheritoko), very good signal, happy to recommend.

https://simpower.com.au/japan-travel-esim-iij-ntt-docomo/

Just FYI ..

Docomo has the widest coverage BUT...
depends on your phone make/model, some of the Docomo(or JP Telco) band/frequency may not be supported by your phone.
Most phone are good band 1,3(city urban area) but those in rural area and inside building, you'll need band 19, 26 or 28, the so called Platinum Band.

Japan telco freq bands.
https://teppeilog.com/mobile_bands_japan_e/