For me it was so far back and my two Siemens Gigaset should be replaced wg. the above-mentioned problems. New phones cost about 50 euros, would be manageable.
Instead, I have but just worried new batteries namely of Varta. These batteries were promptly delivered, no cost 10 euros and were placed in 2 minutes to the Gigaset. The phone now works like new and I am happy to have bought only higher quality batteries. The whole thing is a few months ago and the phones are now becoming available and the battery no longer a problem.