One site, but different content for users from different cities - how not to screw up with search engines?

Question

Search engines

Hello everyone, I would like to implement a feature similar to http://www.dns-shop.ru, technopoint.ru/, 2gis.ru
on our poster site . If the user came from Moscow, we show him the poster and news of Moscow, if from Novosibirsk - the poster and news of Novosibirsk, etc. The URL does not change in any way! www.mysite.ru - one address for any city. There are options mysite.ru/Moscow or moscow.mysite.ru in Runet, but I don’t want to use them for various reasons. The user's city is determined by geoip, but there is also a manual switch in the site header.
Everything would be beautiful, but I want search engines to see all the content for all cities, otherwise 90% of the site will go unnoticed for them and search traffic to the site will drop several times. Alternatively, the site can show search engines a large page with a full poster of all cities, and the user can filter it by the user's city. However, I'm afraid that search engines will track this and pessimize the site in the search results.
I want something more reliable.